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 and naming 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

❗ No coverage uploaded for pull request base (ign-physics2@dcafe3b). Click here to learn what that means.
The diff coverage is n/a.

@@               Coverage Diff               @@
##             ign-physics2     #386   +/-   ##
===============================================
  Coverage                ?   83.15%           
===============================================
  Files                   ?      108           
  Lines                   ?     4210           
  Branches                ?        0           
===============================================
  Hits                    ?     3501           
  Misses                  ?      709           
  Partials                ?        0           

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