[core] Migrate Property Framework API to Java 8

* "The constraint mechanism will be completely unlocked with 7.0.0, since we’ll be migrating our API to Java 8."
* :information_source: Note: There is a branch by @oowekyala that might contain some of the tasks above: https://github.com/pmd/pmd/compare/pmd/7.0.x...oowekyala:pmd:1432-property-framework-upgrade-pmd7
