Gradle 9 update and Spring Boot 2 tests

[beatbrot]

I saw the help-wanted label on #2191 (update to Gradle 9) and took a look at it.

Apart from some minor changes that would be needed, one of the larger issues seems to be that the Spring Boot 2 Gradle plugin that is used in boot2-test is utterly incompatible with Gradle 9.

Issues range from the removed Upload task and the dependency-management plugin being broken to the removed LenientConfiguration#getFiles() method. In short: This can't be fixed with a minor workaround.

Therefore, I'd like to propose two alternatives on how we could continue:

1. Remove the boot2-test subproject. Spring Boot 2 is out of OSS support since June 2023. There is however still enterprise support until June 2029. I would argue that those people could also keep using older spock releases.
2. Do the testing with Gradle TestKit. With Gradle TestKit, we could spin up any Gradle version we want inside a test case and run the "test" task on there. So this would mean that we would create a testcase that runs a "test" task with Gradle TestKit. And this test task would run relevant tests. Then, in the original testcase, we can check if the "test" task ran successfully. We won't get a proper test report. On one side, this is a well-supported solution that should serve us well into the future. On the other side, we introduce quite a bit of indirections which decrease developer experience and maintainability.

Before following any of these paths, I wanted to discuss the given options with you guys. Personally, I would lean towards option one, but I would understand if it is more important for you to properly support Spring Boot 2.

Note: I tried to use composite builds in the hope that an includedBuild could declare its own Gradle version. Unfortunately, that is not the case.

[beatbrot]

I found a cleaner way to handle this issue. See #2202 for details. This discussion is therefore obsolete :)