[CT-3206] Add warning_tag to UnversionedBreakingChange

[jtcohen6]

follow-up to #8384

I would like UnversionedBreakingChange to be flagged as a "warning" in CI:

Right now, that should be as simple as adding a warning_tag to UnversionedBreakingChange

For later: We are inconsistent about the application of warning_tag to the formatted messages of WarnLevel events. Probably this should be more consistent, and maybe handled within the EventManager (instead of each event class individually). That's for later.