ci(android): switch upload-artifact to actions/v4 + retire flutter.yml

Last push failed both android and flutter jobs at workflow-setup time
because the runner couldn't resolve forgejo/upload-artifact@v3
(github.com/forgejo/upload-artifact does not exist; the Forgejo
project hosts on Codeberg and our Gitea runner falls through to
github by default). The canonical Gitea Actions form is
actions/upload-artifact@v4, which act_runner resolves cleanly.

Flutter pipeline is being retired in favor of the native Android
client, so flutter.yml is deleted outright rather than fixed. The
container-image build's release-asset polling (release.yml) will now
graceful-degrade when chasing the legacy minstrel-<TAG>.apk name,
which the comment in ci-requirements.md already documents as
acceptable. Renaming the native APK to drop the -android- infix is
deferred to a follow-up so the cutover is reviewable in isolation.

.gitea/workflows/android.yml

@@ -78,7 +78,7 @@ jobs:
 
       - name: Upload debug APK
         if: github.event_name == 'push' && github.ref == 'refs/heads/main'
-        uses: forgejo/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: minstrel-android-debug-${{ github.sha }}
           path: android/app/build/outputs/apk/debug/app-debug.apk