fix(firebase_ui_oauth_apple): Update provider.dart

[yoman07]

Fix for using shouldUpgradeAnonymous for Apple provider

Description

It fixes problem with updating anonymous account for apple provider.

Related Issues

Problem well described here:
Apple log in inconsistency problem

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.
• All unit tests pass (melos run test:unit:all doesn't fail).
• 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.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.