Update CHANGELOG for 8.40.0

[ci skip]

CHANGELOG

 # Changelog for gitlab-workhorse
 
+## v8.40.0
+
+### Added
+- Add project level route for conan package uploads
+  https://gitlab.com/gitlab-org/gitlab-workhorse/-/merge_requests/558
+
+### Other
+- Refactor uploaders to use different upload strategies
+  https://gitlab.com/gitlab-org/gitlab-workhorse/-/merge_requests/553
+
 ## v8.39.0
 
 ### Fixed

changelogs/unreleased/11679-conan-project-level-route.yml

----
-title: Add project level route for conan package uploads
-merge_request: 558
-author:
-type: added

changelogs/unreleased/sh-refactor-uploaders.yml

----
-title: Refactor uploaders to use different upload strategies
-merge_request: 553
-author:
-type: other