Update CI badges in README

[iche033]

🦟 Bug fix

Summary

updated links: ign - > gz

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

All modified and coverable lines are covered by tests ✅

Comparison is base (303e843) 75.55% compared to head (9bc9ca4) 75.54%.

Additional details and impacted files

@@                Coverage Diff                @@
##           gz-rendering8     #947      +/-   ##
=================================================
- Coverage          75.55%   75.54%   -0.01%     
=================================================
  Files                177      177              
  Lines              16886    16887       +1     
=================================================
  Hits               12758    12758              
- Misses              4128     4129       +1     

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