ci: pin upload-artifact to v3 (Gitea GHES-mode incompatible with v4)

android.yml run 116 (main push on 2534384e) failed the debug-APK
upload step with:

  ::error::@actions/artifact v2.0.0+, upload-artifact@v4+ and
  download-artifact@v4+ are not currently supported on GHES.

Gitea Actions emulates GHES; the v2 backend used by upload-artifact
v4 isn't implemented there yet. Pinning to @v3 keeps the main-push
debug-APK channel working without affecting the tag-push release
path (which doesn't use this action).

.gitea/workflows/android.yml

@@ -85,7 +85,10 @@ jobs:
 
       - name: Upload debug APK
         if: github.event_name == 'push' && github.ref == 'refs/heads/main'
-        uses: actions/upload-artifact@v4
+        # Gitea Actions runs in GHES-emulation mode; @actions/artifact v2+
+        # (i.e. upload-artifact@v4+) errors with "GHESNotSupportedError".
+        # Pin to @v3 until act_runner or the artifact backend catches up.
+        uses: actions/upload-artifact@v3
         with:
           name: minstrel-android-debug-${{ github.sha }}
           path: android/app/build/outputs/apk/debug/app-debug.apk