README: update references from main to gz-physics8

[scpeters]

🦟 Bug fix

Part of https://github.com/gazebo-tooling/release-tools/issues/1092

Summary

Update branch references in the README from main to gz-physics8 including the jenkins badges, an example git command, and a markdown links.

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)
• [ ] 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.