extract-major-beta-version code cleanup

[Mugurell]

Following the recent review from #3 there are some things to improve:

• hardcode releases_ as the prefix of all release branches - Fix #2: Generalize the script to work on any repository. #3 (comment)
• refactor get_latest_beta_major_version method naming to better express what it does - get the major version of any latest release branch (not necessarily beta) - Fix #2: Generalize the script to work on any repository. #3 (comment)

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And following https://github.com/mozilla-mobile/focus-android/actions/runs/2499415451 looks like we still need to update the script to support Focus.
Based on my investigations it seems like the script fails on Focus because Focus uses a "major_version .minor_version" naming scheme for branches while this action hardcodes a "major_version .minor_version .patch_version" naming scheme for branches as Fenix uses.