Update sdf plugins to use actuator_number.

[bperseghetti]

🦟 Bug fix

Fixes #1975

Summary

Updates actuatorNumber and motorNumber in SDF plugin to actuator_number for unified/proper use.

Adds deprecation warning for those using old motorNumber and still passes value to actuator_number.

Checklist

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

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #1976 (e6946a6) into gz-sim7 (0742968) will decrease coverage by 0.10%.
The diff coverage is 72.10%.

❗ Current head e6946a6 differs from pull request most recent head 3c3af7c. Consider uploading reports for the commit 3c3af7c to get more accurate results

@@             Coverage Diff             @@
##           gz-sim7    #1976      +/-   ##
===========================================
- Coverage    65.08%   64.98%   -0.10%     
===========================================
  Files          352      352              
  Lines        28312    28369      +57     
===========================================
+ Hits         18428    18437       +9     
- Misses        9884     9932      +48     

Impacted Files	Coverage Δ
src/SimulationRunner.hh	100.00% <ø> (ø)
...rc/systems/ackermann_steering/AckermannSteering.hh	100.00% <ø> (ø)
include/gz/sim/EventManager.hh	75.75% <66.66%> (-3.41%)	⬇️
...s/multicopter_motor_model/MulticopterMotorModel.cc	76.40% <69.23%> (+0.48%)	⬆️
src/systems/sensors/Sensors.cc	62.64% <70.58%> (-0.37%)	⬇️
...rc/systems/ackermann_steering/AckermannSteering.cc	80.09% <73.52%> (-0.45%)	⬇️
src/SimulationRunner.cc	91.47% <100.00%> (-0.43%)	⬇️

... and 3 files with indirect coverage changes