Add migration notes

Author: christophfroehlich

doc/migration.rst

@@ -3,3 +3,7 @@
 Migration Guides: Jazzy to Kilted
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 This list summarizes important changes between Jazzy (previous) and Kilted (current) releases, where changes to user code might be necessary.
+
+GripperActionController
+*****************************
+The ``effort_controllers/GripperActionController`` and ``position_controllers/GripperActionController`` have been removed. The ``parallel_gripper_action_controller/GripperActionController`` should be used instead. `(#1652 <https://github.com/ros-controls/ros2_controllers/pull/1652>`__).