[packages] Raise iOS min version for deprecation checks #194245

There's an analyze run in flutter/packages that's meant to avoid introducing new deprecated code (other than within an availability-gated fallback path), and the idea is that it's supposed to be set to a higher min iOS version than our actual minimum, so that dropping old iOS versions doesn't suddenly give a bunch of deprecation warnings, and they can instead be addressed proactively.

Currently, for iOS it's still set to iOS 14, which is now lower than our minimum supported version. I ran into this in flutter/packages#13197 where CI tried to block me from removing deprecated code paths that we no longer need, because it's analyzing for iOS 14 instead of 15. I can bump it to 15 there (if that doesn't cause issues in other packages, TBD), but ideally it should be raised to something relatively recent, and any warnings that show up in that process addressed with new codepaths.