feat: Add removal of old search APIs to 33 migration guide

Signed-off-by: Carl Schwan <carl.schwan@nextcloud.com>

Author: CarlSchwan

developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_33.rst

@@ -55,3 +55,6 @@ Removed APIs
 
 - 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.
+- The ``\OCP\Search\PaggedProvider``, ``\OCP\Search\Provider`` and ``\OCP\Search\Result`` classes were
+  deprecated since Nextcloud 20 and were now removed. Instead use ``\OCP\Search\SearchResult`` and
+  ``\OCP\Search\IProvier``, available since Nextcloud 20.