Fix macOS migration nothing-to-upgrade test (#114703)

When testing whether a macOS deployment target version migration is
unnecessary due to already being at the target version, we were
previously checking the iOS deployment target version rather than the
macOS version. This updates it to test MACOSX_DEPLOYMENT_TARGET instead.

Author: cbracken

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

@@ -188,7 +188,7 @@ keep this 2
     });
 
     testWithoutContext('skipped if nothing to upgrade', () {
-      const String xcodeProjectInfoFileContents = 'IPHONEOS_DEPLOYMENT_TARGET = 11.0;';
+      const String xcodeProjectInfoFileContents = 'MACOSX_DEPLOYMENT_TARGET = 10.13;';
       xcodeProjectInfoFile.writeAsStringSync(xcodeProjectInfoFileContents);
 
       final DateTime projectLastModified = xcodeProjectInfoFile.lastModifiedSync();