Require gz-math 6.13 for ign->gz transition

[azeey]

🦟 Bug fix

Fixes https://github.com/gazebosim/gz-physics/issues/595

Summary

We need gz-math 6.13 and newer to use gz namespaces.

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.