com.zulipmobile moved to a new GitHub repository

The source of com.zulipmobile no longer lives in https://github.com/zulip/zulip-mobile, but in https://github.com/zulip/zulip-flutter, cf. https://blog.zulip.com/2025/06/17/flutter-mobile-app-launched/. The application ID stayed the same, cf. https://github.com/zulip/zulip-flutter/blob/57c2471ee8897c0f9c988b9ebab0dd082fa8d902/android/app/build.gradle#L41. https://gitlab.com/fdroid/fdroiddata/-/blob/770edbfbd80fa2a229bf06d5c1d769e66477f884/metadata/com.zulipmobile.yml would have to be adjusted.