Use latest build number/version code when publishing a beta

Currently, the Fastfile parses and increments the build number/version code to increment it, and then do a post release commit on the master branch.

It should use the latest build number/version code available from Testflight/Google Play and use it to generate the new build number/version code.
Doing this will allow us to remove the post commit release on master and so to publish beta builds from any branch.

See https://docs.fastlane.tools/actions/#latest_testflight_build_number and https://docs.fastlane.tools/actions/#google_play_track_version_codes