Update CHANGELOG for 8.55.0

[ci skip]

CHANGELOG

 # Changelog for gitlab-workhorse
 
+## v8.55.0
+
+### Added
+- Add direct upload acceleration for metric images
+  https://gitlab.com/gitlab-org/gitlab-workhorse/-/merge_requests/653
+
+### Fixed
+- Image scaler: add success-client-cache status label
+  https://gitlab.com/gitlab-org/gitlab-workhorse/-/merge_requests/656
+
 ## v8.54.0
 
 ### Changed

changelogs/unreleased/mk-image-scaler-client-cache-label.yml

----
-title: 'Image scaler: add success-client-cache status label'
-merge_request: 656
-author:
-type: fixed

changelogs/unreleased/sarnold-metric-image-upload-route.yml

----
-title: Add direct upload acceleration for metric images
-merge_request: 653
-author:
-type: added