plugin template min flutter sdk version to 1.20 (#62605)

packages/flutter_tools/templates/plugin/pubspec.yaml.tmpl

@@ -6,7 +6,7 @@ homepage:
 
 environment:
   sdk: ">=2.7.0 <3.0.0"
-  flutter: ">=1.17.0 <2.0.0"
+  flutter: ">=1.20.0 <2.0.0"
 
 dependencies:
   flutter:

packages/flutter_tools/test/commands.shard/permeable/create_test.dart

@@ -23,6 +23,7 @@ import 'package:flutter_tools/src/project.dart';
 import 'package:flutter_tools/src/version.dart';
 import 'package:mockito/mockito.dart';
 import 'package:process/process.dart';
+import 'package:pub_semver/pub_semver.dart';
 import 'package:pubspec_parse/pubspec_parse.dart';
 
 import '../../src/common.dart';
@@ -1966,6 +1967,21 @@ void main() {
     Logger: () => logger,
   });
 
+  testUsingContext('newly created plugin has min flutter sdk version as 1.20.0', () async {
+    Cache.flutterRoot = '../..';
+    when(mockFlutterVersion.frameworkRevision).thenReturn(frameworkRevision);
+    when(mockFlutterVersion.channel).thenReturn(frameworkChannel);
+
+    final CreateCommand command = CreateCommand();
+    final CommandRunner<void> runner = createTestCommandRunner(command);
+    await runner.run(<String>['create', '--no-pub', '--template=plugin', projectDir.path]);
+    final String rawPubspec = await projectDir.childFile('pubspec.yaml').readAsString();
+    final Pubspec pubspec = Pubspec.parse(rawPubspec);
+    final Map<String, VersionConstraint> env = pubspec.environment;
+    expect(env['flutter'].allows(Version(1, 20, 0)), true);
+    expect(env['flutter'].allows(Version(1, 19, 0)), false);
+  });
+
 }
 
 Future<void> _createProject(