Increase VERSION and add entry in Changelog

Change-Id: I4f04533c058167e00d74bd0e9327fc358fbd9732
Merge remote-tracking branch 'gerrit/5619_have_an_enhanced_experimental_theme' into HEAD

* gerrit/5619_have_an_enhanced_experimental_theme:
  Increase VERSION and add entry in Changelog

Change-Id: I099535c7e3b73c8fa1405319cde46b975f687e05
This is Tuleap 6.9.99.21

Change-Id: Iefb32a2867428543de1a8541a2172c3edb5ead59