Ignition -> Gazebo

[jennuine]

🦟 Bug fix

Part of:

• https://github.com/gazebosim/gz-sim/issues/1575
• https://github.com/gazebo-tooling/release-tools/issues/698

Summary

Updated links and naming in README.md

Checklist

• [x] 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)
• [x] codecheck passed (See contributing)
• [x] All tests passed (See test coverage)
• [x] 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.