Use new Joint APIs for Parent/Child name

[scpeters]

🦟 Bug fix

Follow-up to gazebosim/sdformat#1053.

Summary

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 #92 (f0d48c2) into main (59a00a7) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #92   +/-   ##
=======================================
  Coverage   95.17%   95.17%           
=======================================
  Files          24       24           
  Lines        1099     1099           
=======================================
  Hits         1046     1046           
  Misses         53       53           

Impacted Files	Coverage Δ
...sdformat_mjcf/mjcf_to_sdformat/converters/joint.py	92.30% <100.00%> (ø)

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