Fix fortress CI by switching from Focal to Jammy

[azeey]

🦟 Bug fix

Summary

The Deploy API Docs workflow has been broken for a while https://github.com/gazebosim/docs/actions/runs/29398576409 due to an error in setup-gazebo action. The reported error is "incompatible Gazebo and Ubuntu combination". I believe this is because the configured OS for Fortress was changed to Ubuntu Jammy when Ubuntu Focal went EOL in gazebo-tooling/release-tools#1428.

Checklist

• Signed all commits for DCO
• Added a screen capture or video to the PR description that demonstrates the fix (as needed)
• 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)
• Updated Bazel files (if adding new files). Created an issue otherwise.
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[azeey]

I did a manual run of the workflow using this branch and the Fortress build has succeded https://github.com/gazebosim/docs/actions/runs/29437378063