fix: improve handling of branch validity

[dsanders11]

This contains a few different changes:

• Reverts Fetch schedule.json with eolGracePeriod=7 #156 as this solves the underlying problem of running /verify-semver on an EOL branch
• Allows EOL branches to be used in all commands if they're specifically named - providing 'all' for the branch name still distills down to just supported branches
• Always checks for the latest potential major branch on GH since /schedule.json is cached - this closes the window where a new release branch has just been cut and the schedule hasn't reflected it yet