feat: Bump Firebase iOS SDK to 8.15.0.

[russellwheatley]

Description

See title.

Related Issues

None.

Checklist

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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.
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Breaking Change

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• Yes, this is a breaking change.
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[github-actions]

Visit the preview URL for this PR (updated for commit 73671c0):

https://flutter-firebase-docs--pr8459-russell-ios-sdk-8-1-biod6mif.web.app

_{(expires Wed, 20 Apr 2022 15:07:34 GMT)}

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[russellwheatley]

Closing as the CLA has been signed for this PR