Promote categories from experimental

[benmccann]

Maven, Gradle, and SBT all have built-in support for categories. Given that this feature has been so widely adopted, I think it makes sense to think about promoting it from the experimental package.

[junit-builds]

JUnit 4 is now in maintenance mode.

At this point, only critical bugs and security issues will be fixed.

The team has therefore decided to close this issue.