[Flathub] Update runtime to 50

https://github.com/flathub/com.ranfdev.Geopard/pull/15
```
$ sudo flatpak update
Looking for updates…

Info: runtime org.gnome.Platform branch 48 is end-of-life, with reason:
   The GNOME 48 runtime is no longer supported as of March 24, 2026. Please ask your application developer to migrate to a supported platform.
Info: applications using this runtime:
   com.ranfdev.Geopard
```