Delete highlights.md

[scpeters]

🦟 Bug fix

Removes unused file

Summary

This is now hosted in release notes in the docs repo.

https://gazebosim.org/docs/jetty/release_notes/

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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• Was GenAI used to generate this PR? If so, make sure to add "Generated-by" to your commits. (See this policy for more info.)

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