[various] Convert remaining Groovy files to Kotlin

[stuartmorgan-g]

This is the last in the series of PRs to convert all of our legacy Groovy Gradle files to Kotlin. This converts all remaining Groovy files in the repository to Kotlin (with the exception of legacy_project; see flutter/flutter#184656). This is the app-facing plugin example apps, as well as the Pigeon example app, as those were the ones not yet converted.

As with #11421 I did this as an in-place update primarily done via Gemini, rather than by recreating the entire test app, since plugins tend to have special setup/configs in their example apps:

• I had Gemini mass-convert the remaining files using the previous conversions as a template.
• I had Gemini specifically re-review the results for any accidental changes.
• I did some manual checks for unexpected diffs between resulting files.
• I did a small amount of manual fixup where Gemini didn't convert correctly.
• I removed settings_aar.gradle from local_auth's example (the only package containing it); it seems to have been something auto-created by an old version of flutter for AndroidX compat during a transition phase, and no longer seems to be relevant.

I also removed all the logic from the repo tool to validate Groovy versions of files, now that we don't need to support them, and simplified/updated the tests accordingly.

Fixes flutter/flutter#176065

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

Footnotes

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. ↩ ↩²

[gemini-code-assist]

Code Review

This pull request migrates Android example projects across numerous packages from Groovy to Kotlin DSL, replacing build.gradle and settings.gradle files with .kts versions. The gradle-check tool and its test suite are updated to remove Groovy-specific logic, hardcoding validation for Kotlin DSL syntax and file naming conventions. I have no feedback to provide as the review comments were found to be factually incorrect regarding the state of the implementation in the provided diffs.

[stuartmorgan-g]

This is now really ready for review.