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

Merging #178 (ce17921) into ign-launch2 (66f0b31) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           ign-launch2     #178   +/-   ##
============================================
  Coverage        30.24%   30.24%           
============================================
  Files                3        3           
  Lines              734      734           
============================================
  Hits               222      222           
  Misses             512      512           

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