chore: cherry-pick various commits from NNBD for stable

[Salakar]

Description

Cherry pick a bug fix for Firestore and a feature release for Auth onto the non-null-safety/stable release branch.

Related Issues

#3565
#4263

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 (flutter 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 (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[google-cla]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

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.

Note to project maintainer: There may be cases where the author cannot leave a comment, or the comment is not properly detected as consent. In those cases, you can manually confirm consent of the commit author(s), and set the cla label to yes (if enabled on your project).

ℹ️ Googlers: Go here for more info.

[dackers86]

@googlebot I consent.

[google-cla]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

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.

Note to project maintainer: There may be cases where the author cannot leave a comment, or the comment is not properly detected as consent. In those cases, you can manually confirm consent of the commit author(s), and set the cla label to yes (if enabled on your project).

ℹ️ Googlers: Go here for more info.

[mikehardy]

@googlebot I consent

[mikehardy]

This is fully tangential but I noticed you've still got a separate "cancel workflow runs" action here https://github.com/FirebaseExtended/flutterfire/runs/1823207554 - like react-native-firebase used to have, which requires hard coding workflow IDs etc., but the cancel workflow action was extended some months (almost a year?) back so that you could put it in the workflow that should be cancelled, and it will auto-magically get the id for that workflow and cancel it, so you can avoid the separate step, and avoid magical workflow IDs in CI.

[mikehardy]

Here is the actionable commit that could be ported: invertase/react-native-firebase@0c35772