Add missing POPULATE_SQUASH_COMMENT_WITH_COMMIT_MESSAGES in app.example.ini (#33363)

Ref:
https://docs.gitea.com/next/administration/config-cheat-sheet?_highlight=populate_squash_comment_with_commit_messages#repository---pull-request-repositorypull-request

custom/conf/app.example.ini

@@ -1129,6 +1129,9 @@ LEVEL = Info
 ;; In default merge messages only include approvers who are official
 ;DEFAULT_MERGE_MESSAGE_OFFICIAL_APPROVERS_ONLY = true
 ;;
+;; In default squash-merge messages include the commit message of all commits comprising the pull request.
+;POPULATE_SQUASH_COMMENT_WITH_COMMIT_MESSAGES = false
+;;
 ;; Add co-authored-by and co-committed-by trailers if committer does not match author
 ;ADD_CO_COMMITTER_TRAILERS = true
 ;;