Update CI badges in README

[iche033]

🦟 Bug fix

Summary

update 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 (8b40b54) 65.62% compared to head (8f10ba4) 65.65%.

Additional details and impacted files

@@             Coverage Diff             @@
##           gz-sim8    #2271      +/-   ##
===========================================
+ Coverage    65.62%   65.65%   +0.03%     
===========================================
  Files          324      324              
  Lines        30924    30924              
===========================================
+ Hits         20293    20303      +10     
+ Misses       10631    10621      -10     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.