docs(firebase_auth): replace deprecated fetchSignInMethodsForEmail in error handling docs

[karnemanideep]

Description

The fetchSignInMethodsForEmail() method is deprecated but the error
handling documentation still referenced it for handling
account-exists-with-different-credential errors. This PR updates
the code example to use the recommended approach instead.

Related Issues

Fixes #12909

Checklist

• 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

• No, this is not a breaking change.

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