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.

• [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• [x] My PR includes unit or integration tests for all changed/updated/fixed behaviors (See [Contributor Guide]).
• [x] All existing and new tests are passing.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] The analyzer (melos run analyze) does not report any problems on my PR.
• [x] All unit tests pass (melos run test:unit:all doesn't fail).
• [x] I read and followed the [Flutter Style Guide].
• [x] I signed the [CLA].
• [x] 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.
• [x] No, this is not a breaking change.

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