[packages] Consider updating legacy_project to Kotlin Gradle #184656

The legacy_project app—which is used to help ensure that plugins still build with old, minimally-updated projects that existing in the wild—is now the only use of Groovy in the repository. Since my understanding is that we don't expect any compat issues related to using Kotlin vs Groovy for plugin files, we may want to consider updating those files to Kotlin Gradle (without modernizing their contents beyond that), so that we don't need to continue to deal with Groovy in one obscure place.

On the other hand, old real-world apps are almost certainly still using Groovy, so if we think it's better to err on the side of minimal changes here, as we usually do for this test app, we may want to leave it as-is.