Mark update-packages as non-experimental (#35467)

Marking it as experimental was breaking tests and packaging
scripts on stable branches.

packages/flutter_tools/lib/src/commands/update_packages.dart

@@ -77,9 +77,6 @@ class UpdatePackagesCommand extends FlutterCommand {
       );
   }
 
-  @override
-  bool get isExperimental => true;
-
   @override
   final String name = 'update-packages';
 

packages/flutter_tools/test/commands/update_packages_test.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_tools/src/commands/update_packages.dart';
+
+import '../src/common.dart';
+import '../src/context.dart';
+
+void main() {
+  group('UpdatePackagesCommand', () {
+    // Marking it as experimental breaks bots tests and packaging scripts on stable branches.
+    testUsingContext('is not marked as experimental', () async {
+      final UpdatePackagesCommand command = UpdatePackagesCommand();
+      expect(command.isExperimental, isFalse);
+    });
+  });
+}