Ignition -> Gazebo

[jennuine]

🦟 Bug fix

Part of:

• Increase test coverage gz-sim#1575
• Rename Ignition to Gazebo gazebo-tooling/release-tools#698

Summary

Updated links, naming, and added badges to match the other Gazebo libraries in README.md

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 #1080 (cd6fbfd) into sdf9 (2f13983) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             sdf9    #1080   +/-   ##
=======================================
  Coverage   87.59%   87.59%           
=======================================
  Files          64       64           
  Lines       10061    10061           
=======================================
  Hits         8813     8813           
  Misses       1248     1248           

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