chore(developer): document removal of global Handlebars for Nextcloud 34

Signed-off-by: Ferdinand Thiessen <opensource@fthiessen.de>

Author: susnux

developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_34.rst

@@ -42,6 +42,7 @@ Removed APIs
   - ``jQuery UI`` was deprecated and scheduled for removal since Nextcloud 19.
   - ``Backbone`` was deprecated and scheduled for removal since Nextcloud 19.
   - ``OC.Files.Client`` as it was extending the ``Backbone``.
+  - ``Handlebars`` was deprecated and scheduled for removal since Nextcloud 19.
 
 
 Back-end changes