Introducing the FinderBase<CandidateType> implementation introduces a lot of things that need to be cleaned up, deprecated, etc. This ticket will act as a followup to #123634 but be purely focused on cleanup, allowing #123634 to focus on cleanly introducing the new API.

Cleanup:

• Deprecate finders.dart#Finder
  • References to Finder should be updated to WidgetFinder
• Deprecate finders.dart#ChainedFinder in lieu of ChainedFinderMixin
• Deprecate finders.dart#MatchFinder in lieu of MatchFinderMixin
• Deprecate the methods in _LegacyFinderMixin and migrate to the FinderBase API internally