Call gz sim directly without shell=True for a clean exit

[azeey]

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses
Primary OS tested on	Ubuntu
Robotic platform tested on	System tests on Gazebo simulation
Does this PR contain AI generated software?	No

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Description of contribution in a few bullet points

• Port system tests in nav2_system_tests #4440 (comment) mentions that Gazebo needs to be SIGKILL'd. This is due to ExecuteProcess with shell=True gets incorrect exit code from the executed process ros2/launch#757 where the SIGINT signal doesn't propagate to Gazebo when shell=True. A simple fix is to execute gz-sim directly instead of using the launch files from ros_gz.

TODO:

• If we're okay with this chance, I can update the rest of the launch files.

For Maintainers:

• Check that any new parameters added are updated in docs.nav2.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists

[SteveMacenski]

Testing this right now with the full suite of tests and working well. I think we'll keep the python3 util in place so that optionality exists (its still generally useful, even outside of GZ, to bring down zombies)

Closing, new PR with the full suite ready. Thanks for the help @azeey!