This PR makes the repo-wide changes from #11669 in a smaller PR.

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Google maps sdk9 readme had a typo where it said sdk10 instead in the title

*List which issues are fixed by this PR. You must list at least one issue.*

## Pre-Review Checklist

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When determining whether a package requires `flutter`, look for any `sdk: flutter` dependency rather than just for the `flutter` dependency in particular. This will ensure that includes, for example, packages with a `flutter_test` dev dependency, but no `flutter` dependency.

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Migrate remaining example apps to built-in kotlin. Refactored validation checks for kotlin configs and added unit tests for example apps. Now both plugin modules and plugin example apps have checks for kotlin configs (includes built-in kotlin configs).

Fixes flutter/flutter#187325
## Pre-Review Checklist

**Note**: The Flutter team is currently trialing the use of [Gemini Code Assist for GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code). Comments from the `gemini-code-assist` bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.