Update changelog entry to reflect that we're renaming

changelogs/unreleased/add-ci_variables-environment_scope-mysql.yml

 ---
-title: Remove duplicated variables with the same key for projects. Add environment_scope
+title: Rename duplicated variables with the same key for projects. Add environment_scope
   column to variables and add unique constraint to make sure that no variables could
   be created with the same key within a project
 merge_request: 12363