Update CHANGELOG for 8.31.0

[ci skip]

CHANGELOG

 # Changelog for gitlab-workhorse
 
+## v8.31.0
+
+### Added
+- Add a signed field on upload requests containing all the workhorse parameters
+  https://gitlab.com/gitlab-org/gitlab-workhorse/-/merge_requests/490
+
+### Other
+- Add automatic changelog generation
+  https://gitlab.com/gitlab-org/gitlab-workhorse/-/merge_requests/484
+
 v 8.30.1
 
 - Sign artifact multipart fields in Workhorse

changelogs/unreleased/261-10io-add-signed-params-to-file-uploads.yml

----
-title: Add a signed field on upload requests containing all the workhorse parameters
-merge_request: 490
-author:
-type: added

changelogs/unreleased/changelog-generator.yml

----
-title: Add automatic changelog generation
-merge_request: 484
-author:
-type: other