Better handling of version updates in AppStream metadata

Currently, every new ScummVM release requires us to manually update the version/release date in the [metainfo patch](https://github.com/flathub/org.scummvm.ScummVM/blob/master/metainfo.patch). We should consider using some placeholder version/date strings and replacing them (using _sed_?) directly in the Flatpak manifest file instead, or updating the version/release date strings directly in upstream.