Add changelog item for merge request versions feature

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
+v 8.12.0 (unreleased)
+  - Add merge request versions !5467
+
 v 8.11.0 (unreleased)
   - Use test coverage value from the latest successful pipeline in badge. !5862
   - Add test coverage report badge. !5708