packages.apt: use jetty package names

[scpeters]

🎉 New feature

Prototype for gazebo-tooling/release-tools#1326.

Summary

Use unversioned gz-jetty-* package names in packages.apt file.

Test it

Verify that CI passes

Requires:

• Provide unversioned jetty package name gazebo-release/gz-cmake5-release#3
• Provide unversioned jetty package names gazebo-release/gz-utils4-release#6

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• 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
• Was GenAI used to generate this PR? If so, make sure to add "Generated-by" to your commits. (See this policy for more info.)

Generated-by: Remove this if GenAI was not used.

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by and Generated-by messages.