CI: Default to flatpak beta branch

[F0bes]

Description of Changes

Changes the daily flatpak branch to beta. When a stable release is cut it will push to the stable branch.

Caveat: The stable branch is going to be stuck on a dev build until the next stable release. I don't if it's ideal to merge this at this time??

Rationale behind Changes

We have a stable version but you can't use it properly with flathub / flatpak because our dev builds are on the same branch.

Suggested Testing Steps

I don't know, I'm scared to commit this :laughing:

resolves #11688