refactor(firebase_database)!: Migrate to sound null-safety

[aaqibismail]

Description

This PR updates the firebase_database package to sound null-safety.

I removed both the flutter_driver and e2e packages because they have not been updated to null-safety yet and from what I've seen the other flutterfire packages have done the same. For certain fields such as the previousSiblingsKey field in the Event class, I was unsure whether to make them nullable or not, so I opted to keep these variables nullable to be safe.

Related Issues

#5207

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[b3nb5n]

Thank you 🙏🏻

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[rrousselGit]

@googlebot I consent.

[Salakar]

Note: version bump on the package and the changelog entry is not required so these should be reverted.

[rrousselGit]

Could you make sure that the CI pass? thanks!

[aaqibismail]

Could you make sure that the CI pass? thanks!

What should I do about the e2e package. It’s not updated to null safety so the CI is throwing an error.

[google-cla]

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We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

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[Salakar]

Refactored tests to use drive package that other FlutterFire example apps are using. Additionally upgraded the Android example app to v2 embedding to resolve the build issues from CI tested locally;

[Salakar]

@googlebot I consent.