CH7 option is no longer singly applicable

#### Issue details

This is no longer relevant as now all AUX channels have these options and any can be used to trigger the camera.
Initial Setup > Optional Hardware > Camera Gimbal

#### Version
V. 1.3.xx

#### Platform
[X] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
Plane, Copter

#### Hardware type
Pixhawk, Cube

#### Logs
N/A

Start in Mission Planner's Initial Setup > Optional Hardware > Camera Gimbal screen and compare the CH7 wording with the attached screenshot. Update the outdated single-channel guidance so it reflects that any AUX channel can trigger the camera, then verify the setup screen displays the corrected information.

Written by the indexing model from the issue text.