gettext-flatpak: Add a parameter to specify push branch

### Problem

gettext-flatpak action seems to try pushing to the default branch at the moment, so it can be blocked as violations if the repository forbids direct push to the branch via a branch protection rule.

### Proposal

Add a parameter to `gettext-flatpak/action.yml` to specify a branch where to push to.

### Prior Art (Optional)

_No response_