Fixes #98608

Rather than requiring the user to know the correct increment level for the next release version, instead have the tool auto-determine the next version with the algorithm:

1. if the tip of the candidate branch is also on master, that means this is the first release on the branch; compute the version from the candidate branch (this would fail if the candidate branch was named wrong) and tag the branch point, then increment the n for the actual release
2. else if the channel is stable, the increment should be z (the tool already has logic to "increment" to 0 if this is the first stable release)
3. else (i.e. channel is beta), increment the n

Also, provide an optional --version-override command-line option, for situations where we need to tell the tool exactly which version to use.