chore: Replace FallThroughError with UnsupportedError

[dcharkes]

Description

FallThroughError is deprecated and will be removed with Dart 3.0. Since this rolls into g3, it's blocking us landing the removal.

This PR migrates the errors to UnsupportedErrors.

Related Issues

• [breaking change] Discontinue deprecated core library APIs, part II dart-lang/sdk#49529
• 🐛 [auth][firestore][database] Remove use of deprecated FallThroughError API #10079

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.

Users should not catch Errors, only Exceptions. As such this is not a breaking change.

[dcharkes]

Duplicate of:

• refactor!: remove FallThroughError before deletion in Dart 3 #10081

Feel free to merge that one and close this one.

[dcharkes]

Thanks @Lyokone !

[dcharkes]

I don't have merge powers here, so please go ahead and merge it.

[dcharkes]

Thanks @rrousselGit !

[itsjustkevin]

Hey @dcharkes for a sanity check, does this PR close #10079?

[dcharkes]

Yes