Merge pull request #13645 from nextcloud/feat/document-ijob-execute-removal

feat(developer): document removal of `IJob::execute`

Author: susnux

developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_33.rst

@@ -52,4 +52,5 @@ Deprecated APIs
 Removed APIs
 ^^^^^^^^^^^^
 
-- TBD
+- The ``\OCP\BackgroundJob\IJob::execute`` method was deprecated since Nextcloud 25 and was now removed.
+  Instead use the ``IJob::start`` method, available since Nextcloud 25.