README: update references from main to gz-physics8

[scpeters]

🦟 Bug fix

Part of gazebo-tooling/release-tools#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

• 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.