Remove "(unreleased)" for 8.6.3

[ci skip]
Signed-off-by: Rémy Coutable <remy@rymai.me>

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
-v 8.6.3 (unreleased)
+v 8.6.3
   - Destroy related todos when an Issue/MR is deleted. !3376
   - Fix error 500 when target is nil on todo list. !3376
   - Fix copying uploads when moving issue to another project. !3382