Harmonic: disable Ubuntu Focal CI

[scpeters]

🦟 Bug fix

Follow-up to gazebo-tooling/release-tools#1428

Summary

Ubuntu Focal is already EOL, so switch to Jammy.

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)
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