`upgrade_icechunk_repository` with dry_run=True needs to warn that you haven't actually upgraded

[ianhi]

Otherwise there is no feedback that you haven't, and you can accidentally think you did!

[dcherian]

In general, I am against such warnings that warn users of a choice they made; but it seems the default at the moment is dry_run=True.

How about we set the default to None? If None we warn and set to True. If a user specifies True then no warning is raised.

[ianhi]

Agreed. the issue was that I thought I had made a choice to upgrade, and was surprised when I wasn't allowed to amend.