[google_sign_is] Update iOS SDK to 8.0

[stuartmorgan-g]

Updates the SDK version from 7.1 to 8.0, to pick up Firebase App Check support.

The breaking change in the SDK is because it dropped iOS 11 support, which this plugin already didn't support.

Fixes flutter/flutter#166967

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I linked to at least one issue that this PR fixes in the description above.
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Footnotes

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests or get an explicit test exemption before merging.

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

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[stuartmorgan-g]

test-exempt: rolling dependencies

[vashworth]

LGTM