Add missing CHANGELOG entry

[ci skip]
Signed-off-by: NRémy Coutable <remy@rymai.me>

CHANGELOG

@@ -22,6 +22,7 @@ v 8.12.0 (unreleased)
   - Use Search::GlobalService.new in the `GET /projects/search/:query` endpoint
   - Fix pagination on user snippets page
   - Fix sorting of issues in API
+  - Sort project variables by key. !6275 (Diego Souza)
   - Ensure specs on sorting of issues in API are deterministic on MySQL
   - Escape search term before passing it to Regexp.new !6241 (winniehell)
   - Fix pinned sidebar behavior in smaller viewports !6169