[core] Stabilize tree export API

The API around [TreeRenderer](https://github.com/pmd/pmd/blob/master/pmd-core/src/main/java/net/sourceforge/pmd/util/treeexport/TreeRenderer.java) has been `@Experimental` since it was introduced in PMD 6.21.0 (#2216). I think it's been rather stable and we could promote it to a stable API.