Fix SDF plugin variables for MulticopterMotorModel

[bperseghetti]

🦟 Bug fix

Fixes #1980

Summary

Updated the MulticopterMotorModel plugin to use unified SDF variable formatting. Included a deprecation warning for users that the old variables will be removed in harmonic.

Checklist

• [x] Signed all commits for DCO
• [x] Added tests
• [x] Updated documentation (as needed)
• [x] Updated migration guide (as needed)
• [x] Consider updating Python bindings (if the library has them)
• [x] codecheck passed (See contributing)
• [ ] All tests passed (See test coverage)
• [x] While waiting for a review on your PR, please help review another open pull request to support the maintainers

[azeey]

FWIW, the parameter names were directly taken from https://github.com/ethz-asl/rotors_simulator/blob/master/rotors_gazebo_plugins/src/gazebo_motor_model.cpp to make it easier for users to transition from RotorS based models.

[azeey]

The plan is to make backward compatible changes in ign-gazebo6 and gz-sim7 without warnings (maybe with a todo to enable warnings) and then enable warnings in main (Harmonic). And then remove the old parameters in Gazebo-I

[azeey]

Removing beta from this as well per our conversation with Ben.