ci(audience): align mobile-build Unity 6 cells with playmode at 6000.4.0f1

- Changes the mobile-build matrix Unity 6 entry from 6000.4.5f1 to 6000.4.0f1.
- Matches the version used by the playmode (Win/macOS) and playmode-linux jobs.
- Removes the stale comment that GameCI does not publish a 6000.4.0f1 Docker image.
- Android and Linux IL2CPP tags for 6000.4.0f1 exist on docker.io/unityci/editor.
- iOS variant is not pre-verified; CI will confirm or fail this cell.
- Addresses Cursor Bugbot comment on PR #745, link in the PR description.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ImmutableJeffrey

.github/workflows/test-audience-sample-app.yml

@@ -467,8 +467,6 @@ jobs:
   # containers so self-hosted macOS/Windows machines are not occupied.
   # Scope: IL2CPP compile pipeline only. Runtime tests require a real device and
   # are out of scope until a device farm is available.
-  # Note: Unity 6 cells use 6000.4.5f1 (the latest patch with a GameCI image);
-  # 6000.4.0f1 does not have a published GameCI Docker image.
   mobile-build:
     if: github.event.pull_request.head.repo.fork == false || github.event_name == 'workflow_dispatch'
     name: ${{ matrix.target }} / IL2CPP / Unity ${{ matrix.unity }}
@@ -477,7 +475,7 @@ jobs:
       fail-fast: false
       matrix:
         target: [Android, iOS]
-        unity: ['2021.3.45f2', '2022.3.62f2', '6000.4.5f1']
+        unity: ['2021.3.45f2', '2022.3.62f2', '6000.4.0f1']
         include:
           - target: Android
             method: AndroidBuilder.Build