add maven cache config to gitlab CI

https://blog.exxeta.com/en/2018/04/04/using-cache-with-gitlab-ci-and-speed-up-the-maven-build/

changed milestone to %v15.0.0~alpha.2

added performance refactor labels

mentioned in merge request !1040 (merged)

created merge request !1040 (merged) to address this issue

closed via merge request !1040 (merged)

mentioned in commit 848ce12d