Enable Gradle

[murdos]

This is a meta issue to track what needs to be done before we can publish a first release with Gradle enabled.

Blocking issues:

• [ ] #8588
  • [x] #8963
• [x] #8625
• [x] #9052
• [ ] #8627
• [ ] #8628
• [x] #8629
• [ ] test more modules with gradle in CI tests (with the help of #8986)
• [ ] remove gradle-java from jhlite-hidden-resources.slugs in application*.yml of generator

Issues that can be done later:

• [x] #4956
• [x] #5091