refactor(*): migrate remaining examples & e2e tests to null-safety

[pr-Mais]

Description

Migrate all example apps and e2e tests to null-safety and remove //@dart=2.9.

Included plugins:

• Core
• Auth
• Storage
• Functions
• Firestore
• Messaging
• Crashlytics
• Remote Config
• App Check

Not included:

• Dynamic Links (reworked in refactor(dynamic_links)!: rework as part of #6979 #7263)
• Performance (required in refactor(performance)!: rework as part of #6979 #7391)
• Database (reworked in refactor(database)!: rework as part of #6979 #7202)
• In-App Messaging (done in feat(firebase_in_app_messaging)!: federate & rework plugin #7345)
• ML Custom (to be reworked)
• ML Vision (deprecated)
• AdMob (deprecated)

Related Issues

None.

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.
• 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.

[pr-Mais]

TODO: some plugins still running 0 tests