Document how withPluginClasspath is broken

**Is your feature request related to a problem? Please describe.**

I've been successfully using `withPluginClasspath` for years now, so I'm really wondering what's wrong with it.

**Describe the solution you'd like**

Any kind of documentation other than stating that "it's broken". For example a link to Gradle issue, or blog post.

https://github.com/autonomousapps/dependency-analysis-gradle-plugin/blob/6df0bfa7166832bd2468ec9482f8d48dbeae577e/testkit/README.asciidoc#L5

**Additional context**

I got here from [`WithPluginClasspathUsage`](https://googlesamples.github.io/android-custom-lint-rules/checks/WithPluginClasspathUsage.md.html) which just links to your plugin:

> Avoid usage of GradleRunner#withPluginClasspath, which is broken. Instead use something like https://github.com/autonomousapps/dependency-analysis-gradle-plugin/tree/main/testkit#gradle-testkit-support-plugin

Start with the linked testkit/README.asciidoc section and compare its statement about GradleRunner#withPluginClasspath with the referenced WithPluginClasspathUsage page. Find an authoritative Gradle issue or blog post explaining the problem, then update the documentation with that explanation or link; done means it no longer only says that the feature is broken.

Written by the indexing model from the issue text.