coarsen: better keep_attrs

[mathause]

• Part of where should keep_attrs be set in groupby, resample, weighted etc.? #4513 (maybe the last one - need to double check)
• Tests added
• Passes pre-commit run --all-files
• User visible changes (including notable bug fixes) are documented in whats-new.rst

As per #3891 (comment) I also changed the default to True.

[max-sixty]

Great! Thanks @mathause !