Update application id to `net.librewolf`

Since we now own the domain librewolf.net (see librewolf-community/website#8 (closed)), in the long term the Application ID of LibreWolf should probably be changed from io.gitlab.librewolf-community to net.librewolf.

This is something that requires changes in this repository, in the AppImage repository and in the Flatpack repository and could easily break things, so it probably won't be that easy.