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-gui3@f356192). Click here to learn what that means.
The diff coverage is n/a.

❗ Current head a5107e5 differs from pull request most recent head 4070195. Consider uploading reports for the commit 4070195 to get more accurate results

@@             Coverage Diff             @@
##             ign-gui3     #435   +/-   ##
===========================================
  Coverage            ?   66.42%           
===========================================
  Files               ?       29           
  Lines               ?     3291           
  Branches            ?        0           
===========================================
  Hits                ?     2186           
  Misses              ?     1105           
  Partials            ?        0           

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