Setup: Camera Gimbal page greyed out with AP 4.3 (and higher)

I thought I created an issue for this already but I can't find it so I'm recreating it.

For AP 4.3 (and higher) we've renamed the MNT_ parameters to MNT1_. We should update the Camera Gimbal page to use either the old names or the new names.

I have a partially complete PR here https://github.com/ArduPilot/MissionPlanner/pull/3013

Start with the Mission Planner Camera Gimbal page and inspect PR #3013 to understand the partially completed change. Verify how the page handles the renamed MNT_ and MNT1_ parameters, then confirm that it is no longer greyed out when used with AP 4.3 and higher.

Written by the indexing model from the issue text.