[CP-beta] Increase the recommended Xcode version to Xcode 15 (#146440)

This pull request is created by [automatic cherry pick workflow](https://github.com/flutter/flutter/wiki/Flutter-Cherrypick-Process#automatically-creates-a-cherry-pick-request)
Please fill in the form below, and a flutter domain expert will evaluate this cherry pick request.

### Issue Link:
What is the link to the issue this cherry-pick is addressing?

Part of https://github.com/flutter/flutter/issues/144582#issuecomment-2040653877

### Changelog Description:
Increase the `flutter doctor` recommended Xcode version to Xcode 15.

### Impact Description:

Update the `flutter doctor` validator to suggest Xcode 15 if installed version is lower.

This is to encourage upgrades, as Apple will soon be enforcing this during iOS App Store submissions.
> Please note that as of April 2024 all iOS and iPadOS apps submitted to the App Store must be built with a minimum of Xcode 15 and the iOS 17 SDK.

https://developer.apple.com/ios/submit/

It's also to encourage updates before Swift Package Manager requires this version in https://github.com/flutter/flutter/pull/146256.

### Workaround:
N/A

### Risk:
What is the risk level of this cherry-pick?

### Test Coverage:
Are you confident that your fix is well-tested by automated tests?

### Validation Steps:
Install and `xcode-select` Xcode 14.x or lower.  Run `flutter doctor`.  See 
```
[!] Xcode - develop for iOS and macOS (Xcode 15.3)
    ✗ Flutter requires Xcode 14 or higher.
      Download the latest version or update via the Mac App Store.
```

packages/flutter_tools/lib/src/macos/xcode.dart

@@ -24,7 +24,7 @@ Version get xcodeRequiredVersion => Version(14, null, null);
 
 /// Diverging this number from the minimum required version will provide a doctor
 /// warning, not error, that users should upgrade Xcode.
-Version get xcodeRecommendedVersion => xcodeRequiredVersion;
+Version get xcodeRecommendedVersion => Version(15, null, null);
 
 /// SDK name passed to `xcrun --sdk`. Corresponds to undocumented Xcode
 /// SUPPORTED_PLATFORMS values.

packages/flutter_tools/test/general.shard/macos/xcode_test.dart

@@ -312,12 +312,12 @@ void main() {
           expect(xcode.isRecommendedVersionSatisfactory, isFalse);
         });
 
-        testWithoutContext('version checks pass when version meets minimum', () {
+        testWithoutContext('version checks pass when version meets minimum but not recommended', () {
           xcodeProjectInterpreter.isInstalled = true;
           xcodeProjectInterpreter.version = Version(14, null, null);
 
           expect(xcode.isRequiredVersionSatisfactory, isTrue);
-          expect(xcode.isRecommendedVersionSatisfactory, isTrue);
+          expect(xcode.isRecommendedVersionSatisfactory, isFalse);
         });
 
         testWithoutContext('version checks pass when major version exceeds minimum', () {
@@ -325,7 +325,6 @@ void main() {
           xcodeProjectInterpreter.version = Version(15, 0, 0);
 
           expect(xcode.isRequiredVersionSatisfactory, isTrue);
-          expect(xcode.isRecommendedVersionSatisfactory, isTrue);
         });
 
         testWithoutContext('version checks pass when minor version exceeds minimum', () {
@@ -333,13 +332,35 @@ void main() {
           xcodeProjectInterpreter.version = Version(14, 3, 0);
 
           expect(xcode.isRequiredVersionSatisfactory, isTrue);
-          expect(xcode.isRecommendedVersionSatisfactory, isTrue);
         });
 
         testWithoutContext('version checks pass when patch version exceeds minimum', () {
           xcodeProjectInterpreter.isInstalled = true;
           xcodeProjectInterpreter.version = Version(14, 0, 2);
 
+          expect(xcode.isRequiredVersionSatisfactory, isTrue);
+        });
+
+        testWithoutContext('version checks pass when major version exceeds recommendation', () {
+          xcodeProjectInterpreter.isInstalled = true;
+          xcodeProjectInterpreter.version = Version(16, 0, 0);
+
+          expect(xcode.isRequiredVersionSatisfactory, isTrue);
+          expect(xcode.isRecommendedVersionSatisfactory, isTrue);
+        });
+
+        testWithoutContext('version checks pass when minor version exceeds recommendation', () {
+          xcodeProjectInterpreter.isInstalled = true;
+          xcodeProjectInterpreter.version = Version(15, 3, 0);
+
+          expect(xcode.isRequiredVersionSatisfactory, isTrue);
+          expect(xcode.isRecommendedVersionSatisfactory, isTrue);
+        });
+
+        testWithoutContext('version checks pass when patch version exceeds recommendation', () {
+          xcodeProjectInterpreter.isInstalled = true;
+          xcodeProjectInterpreter.version = Version(15, 0, 2);
+
           expect(xcode.isRequiredVersionSatisfactory, isTrue);
           expect(xcode.isRecommendedVersionSatisfactory, isTrue);
         });

packages/flutter_tools/test/general.shard/macos/xcode_validator_test.dart

@@ -77,7 +77,7 @@ void main() {
       final ProcessManager processManager = FakeProcessManager.any();
       final Xcode xcode = Xcode.test(
         processManager: processManager,
-        xcodeProjectInterpreter: XcodeProjectInterpreter.test(processManager: processManager, version: Version(12, 4, null)),
+        xcodeProjectInterpreter: XcodeProjectInterpreter.test(processManager: processManager, version: Version(14, 4, null)),
       );
       final XcodeValidator validator = XcodeValidator(
         xcode: xcode,
@@ -87,8 +87,8 @@ void main() {
       final ValidationResult result = await validator.validate();
       expect(result.type, ValidationType.partial);
       expect(result.messages.last.type, ValidationMessageType.hint);
-      expect(result.messages.last.message, contains('Flutter recommends a minimum Xcode version of 13'));
-    }, skip: true); // [intended] Unskip and update when minimum and required check versions diverge.
+      expect(result.messages.last.message, contains('Flutter recommends a minimum Xcode version of 15'));
+    }, skip: false); // [intended] Skip this test when minimum and required check versions converge.
 
     testWithoutContext('Emits partial status when Xcode EULA not signed', () async {
       final ProcessManager processManager = FakeProcessManager.list(<FakeCommand>[