Make halt motion act like a brake

[nkoenig]

🦟 Bug fix

Summary

Halt motion would leave joint velocity in place, which would allow a robot to continue moving. This PR makes HaltMotion act like a brake.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• 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

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[codecov]

Codecov Report

Merging #830 (2998859) into ign-gazebo4 (996684c) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           ign-gazebo4     #830      +/-   ##
===============================================
+ Coverage        65.73%   65.77%   +0.03%     
===============================================
  Files              241      241              
  Lines            17839    17841       +2     
===============================================
+ Hits             11727    11735       +8     
+ Misses            6112     6106       -6     

Impacted Files	Coverage Δ
src/systems/physics/Physics.cc	71.68% <100.00%> (+0.06%)	⬆️
src/SimulationRunner.cc	94.01% <0.00%> (+1.05%)	⬆️

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