Upgrade Syncthing to v1.28.0

• Update the bundled syncthing to v1.28.0
• Info.plist: Use string type instead of implicit bool for LSUIElement
• Some github actions continuous integration housekeeping. macos-12 builder deprecated, now using macos-latest. See actions/runner-images#10721.

For full list of changes see: [syncthing-macos/compare/v1.27.12-1...v1.28.0-1](https://github.com/syncthing/syncthing-macos/compare/v1.27.12-1...v1.28.0-1)