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

• https://github.com/ros-navigation/navigation2/pull/4440#issuecomment-2178985584 mentions that Gazebo needs to be SIGKILL'd. This is due to https://github.com/ros2/launch/issues/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
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• [ ] If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists