FIX: suppress offset text in ConciseDateFormatter when largest scale is in years

[harrymknight]

PR Summary

Currently ConciseDateFormatter includes redundant offset text when largest scale is in months or years. This PR suppresses this behaviour and closes #19118.

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

[jklymak]

Yeah I'd add a unit test, but it could just check that the offset text is suppressed rather than being an image test. Thanks!

[jklymak]

Looks good to me. Thanks a lot.

The only minor suggestion I would have is that you could do all the datetime stuff with datetime64, but that is really a matter of taste

[QuLogic]

Thanks @harrymknight! Congratulations on your first PR to Matplotlib 🎉 We hope to hear from you again.

[jklymak]

Yes, thanks @harrymknight !