Manually update 12.6.0 changelog with missing entries

These both ended in .yaml, not .yml, and so weren't picked up by the
changelog generator.

CHANGELOG.md

@@ -238,7 +238,7 @@ entry.
 - Skip updating LFS objects in mirror updates if repository has not changed. !21744
 - Add indexes on deployments to improve environments search. !21789
 
-### Added (117 changes, 16 of them are from the community)
+### Added (119 changes, 18 of them are from the community)
 
 - Add upvote/downvotes attributes to GraphQL Epic query. !14311
 - Delete kubernetes cluster association and resources. !16954
@@ -357,6 +357,8 @@ entry.
 - Added migration which adds service desk username column. !21733
 - Add SentryIssue table to store a link between issue and sentry issue. !37026
 - Add path based targeting to broadcast messages.
+- Add allow failure in pipeline webhook event. !20978 (Gaetan Semet)
+- Add runner information in build web hook event. !20709 (Gaetan Semet)
 
 ### Other (51 changes, 28 of them are from the community)
 

changelogs/unreleased/20709-add-runner-info-in-build-event.yaml

----
-title: Add runner information in build web hook event
-merge_request: 20709
-author: Gaetan Semet
-type: added

changelogs/unreleased/20978-add-allow-failure-in-pipeline-event.yaml

----
-title: |
-  Add allow failure in pipeline webhook event
-merge_request: 20978
-author: Gaetan Semet
-type: added