Backport #2231 to garden: use sdf FindElement API to avoid const_cast

[scpeters]

🦟 Bug fix

Backport #2231 to garden: use sdf FindElement API to avoid const_cast

Summary

Several systems use const_cast in order to call
sdf::Element::GetElement with const ElementPtrs,
but the FindElement API can be used instead.

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 Rebase-and-Merge.

[codecov]

Codecov Report

Merging #2236 (d53bc0f) into gz-sim7 (e88a46f) will decrease coverage by 0.03%.
The diff coverage is 90.90%.

❗ Current head d53bc0f differs from pull request most recent head 955933f. Consider uploading reports for the commit 955933f to get more accurate results

@@             Coverage Diff             @@
##           gz-sim7    #2236      +/-   ##
===========================================
- Coverage    64.77%   64.75%   -0.03%     
===========================================
  Files          357      357              
  Lines        29144    29138       -6     
===========================================
- Hits         18877    18867      -10     
- Misses       10267    10271       +4     

Files	Coverage Δ
src/systems/diff_drive/DiffDrive.cc	83.57% <100.00%> (-0.06%)	⬇️
...stems/joint_state_publisher/JointStatePublisher.cc	83.73% <100.00%> (-0.14%)	⬇️
...ms/joint_traj_control/JointTrajectoryController.cc	78.80% <100.00%> (-0.07%)	⬇️
src/systems/log/LogRecord.cc	79.15% <100.00%> (-0.09%)	⬇️
src/systems/velocity_control/VelocityControl.cc	90.07% <100.00%> (-0.08%)	⬇️
src/systems/tracked_vehicle/TrackedVehicle.cc	46.22% <75.00%> (-0.17%)	⬇️

... and 4 files with indirect coverage changes