Fix dart pub cache clean command on pub.dart (#128171)

Command instruction for clearing dart pub cache is somewhat wrong. Instead of `clear`, `clean` is the correct one. Ref: https://dart.dev/tools/pub/cmd/pub-cache

Fixes https://github.com/flutter/flutter/issues/128663

(Edited by @andrewkolos. Changed "related to" issue to "fixes" to link this PR to the issue).

packages/flutter_tools/lib/src/dart/pub.dart

@@ -630,7 +630,7 @@ class _DefaultPub implements Pub {
         _logger.printStatus('''
 Found an existing Pub cache at $pubCachePath.
 It can be repaired by running `dart pub cache repair`.
-It can be reset by running `dart pub cache clear`.''');
+It can be reset by running `dart pub cache clean`.''');
       }
     }
     final String? home = _platform.environment['HOME'];

packages/flutter_tools/test/general.shard/dart/pub_get_test.dart

@@ -940,6 +940,8 @@ exit code: 66
         context: PubContext.flutterTests);
     expect(logger.statusText,
         contains('Found an existing Pub cache at /global/.pub-cache'));
+    expect(logger.statusText,
+        contains('It can be reset by running `dart pub cache clean`'));
     expect(
       logger.statusText,
       contains(