refactor(firebase_app_check): Disable Jetifier

[vbuberen]

Description

Jetifier is needed to provide compatibility between old support packages and modern androidx ones: https://developer.android.com/tools/jetifier
If project has no support libraries in its dependencies it makes no sense to have Jetifier enabled as it influences the build speed:
https://adambennett.dev/2020/08/disabling-jetifier/
https://developer.android.com/build/optimize-your-build#disable-the-jetifier-flag

Thus, dropping Jetifier. Will open a series of PRs on this topic to not change all packages at once.

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.
• 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 ///).
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• I read and followed the Flutter Style Guide.
• I signed the CLA.
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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.

[vbuberen]

Can be closed in favor of #11699 which contains the same change