[firebase_auth] Fix iOS updatePhoneNumberCredential crash

[daikini]

Description

The updatePhoneNumberCredential on iOS was attempting to get the credentials from the call.arguments instead of the getCredential method and was crashing because the arguments weren't in the format expected. This PR gets the credential from the getCredential method instead of trying to access them directly in the call.arguments.

Related Issues

None

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.

• 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 (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

None

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[collinjackson]

I'd like to get more integration test coverage for this one, but we don't have a test for phone auth yet and I don't want to hold back this fix.