chore: add app_api note to Critical Changes for v31

Signed-off-by: Josh <josh.t.richards@gmail.com>

Author: joshtrichards

admin_manual/release_notes/upgrade_to_31.rst

@@ -51,3 +51,13 @@ Previews
 
 Starting with Nextcloud 31.0.10, the preview provider for MP3 files, which reads cover images embedded in the files, is disabled by default for performance and stability reasons.
 See :doc:`../configuration_files/previews_configuration` for details on how to enable or disable the preview provider.
+
+
+AppApi (app_app) is now a default app
+-------------------------------------
+
+Starting with Nextcloud 30.0.1, the AppApi app is included and enabled by default. See :doc:`../exapps_management` for details.
+
+This app can be disabled in the standard manner via the *Apps* menu if you do not expect to use AppApi integrations in the near future. 
+
+When it is disabled, other apps thet depend on AppApi will not be visible in the app store. AppApi related setup checks will also be deactivated.