release: add step to change each milestone's bug status as "Fix released"

Author: maykathm

RELEASE.md

@@ -615,3 +615,5 @@ Prerequisites:
 Request that snapd QA promotes snapd to stable. The release is done progressively. Once the progressive release has completed, update the [snapd roadmap](https://forum.snapcraft.io/t/the-snapd-roadmap/1973).
 
 #### 2. Request move to `-updates`
+
+Once the move is complete, change all milestone bugs to "Fix released."