fix(auth): deprecate updateEmail() & fetchSignInMethodsForEmail()

[russellwheatley]

Description

iOS, android and web have updated to deprecate updateEmail() & fetchSignInMethodsForEmail() API.

Web: https://firebase.google.com/docs/reference/js/auth.md#fetchsigninmethodsforemail
android: https://firebase.google.com/docs/reference/android/com/google/firebase/auth/FirebaseAuth#fetchSignInMethodsForEmail(java.lang.String)

iOS: https://firebase.google.com/support/release-notes/ios#version_10190_-_dec_5_2023

Related Issues

closes #12121

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.