README: update CI badges, fix expected joint limits

[scpeters]

🦟 Bug fix

Update CI badges

Summary

Point to CI badges for gz-physics6 branch.

I've made this from a specially named branch in order to test an sdformat branch:

• osrf/homebrew-simulation@9507869
• gazebosim/sdformat@d057b07

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 #401 (a65c020) into gz-physics6 (58b3df4) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           gz-physics6     #401   +/-   ##
============================================
  Coverage        76.81%   76.81%           
============================================
  Files              128      128           
  Lines             5737     5737           
============================================
  Hits              4407     4407           
  Misses            1330     1330           

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[scpeters]

@osrf-jenkins run tests please