Update CHANGELOG.md for 12.1.2

[ci skip]

CHANGELOG.md

@@ -4,6 +4,10 @@ entry.
 ## 12.1.2
+### Security (1 change)
+
+- Use source project as permissions reference for MergeRequestsController#pipelines.
+
 ### Security (9 changes)
 - Restrict slash commands to users who can log in.

changelogs/unreleased/security-mr-pipeline-permissions.yml

----
-title: Use source project as permissions reference for MergeRequestsController#pipelines
-merge_request:
-author:
-type: security