fix plugins README google-services dependency from 4.3.0 to 4.2.0

[BondarenkoStas]

Description

README to 5 plugins (firebase_auth, firebase_crashlytics, firebase_in_app_messaging, firebase_messaging, firebase_remote_config) contain Android integration instruction.
Using classpath 'com.google.gms:google-services:4.3.0 fails gradle builds as not supported. Using classpath 'com.google.gms:google-services:4.2.0 solves the problem, but many people struggle for nothing before they actually find it. I perform initial issues triage in flutter/flutter and encounter this issue too many times already. This PR doesn't fix this incompatibility at all, but at least doesn't mislead people.

Related Issues

In general this upvoted comment prevented a lot of issues. Some of them we close as duplicates.

These are examples of resolved issues.
flutter/flutter#37081
flutter/flutter#38163
flutter/flutter#41492
flutter/flutter#41769

Checklist

I don't think there may be possible any tests. The PR is so trivial that almost all the statements below are irrelevant, so just marking them.

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.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• If the pull request affects only one plugin, the PR title starts with the name of the plugin in brackets (e.g. [cloud_firestore])
• 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 updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• 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.

[FaisalAbid]

Thank you for this, I was stuck for hours trying to figure out what was wrong.

[workerbee22]

I think they all now work with google servers 4.3.3 and all the others have moved to this version it seems. So might be better than going backwards

But agree the documented version should work. Although I'm sure all the others are now avoiding any versions of google services mentioned in the documentation.