Undeprecate use of commandline flags

[azeey]

Summary

I'm writing a migration guide for Gazebo classic users and using the command line flags since that's what's available in Humble. It's also very closely matched with gazebo_ros_pkgs so the migration is a lot easier if we keep the flags. I think it would be best to keep these flags, at least for another ROS release cycle.

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.

[azeey]

Do you mind to create a TODO for the next release ?

Done in https://github.com/gazebosim/ros_gz/pull/491/commits/7aa366b64cd4c0e6cac5f183735a4f61faef95be