Fix publishing only top level model pose in pose publisher (backport #2697)

[mergify]

🦟 Bug fix

Fixes #2690

Summary

The pose publisher system should now publish top level model pose with these settings:

        <publish_model_pose>true</publish_model_pose>
        <publish_nested_model_pose>false</publish_nested_model_pose>

This PR preserves the behavior and backward compatibility logic that were added in #1342, i.e.

1. The system should only publish nested model pose with these params:

           <publish_model_pose>false</publish_model_pose>
           <publish_nested_model_pose>true</publish_nested_model_pose>

2. Backward behavior - if <publish_model_pose> is not specified, it defaults to the same value as <publish_nested_model_pose> - we should remove this behavior in gz-sim10 / jetty. I added a todo note for this.

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.

---

This is an automatic backport of pull request #2697 done by [Mergify](https://mergify.com).