Gravity set command fixed (backport #3189)

[mergify]

🦟 Bug fix

Summary

This pull requests solves the issue in gz-physics but since it requires changes in this repo instead of gz-physics I am opening this pr here.
With this gazebo now supports changing gravity at runtime by running
'gz service -s /world/world_demo/set_physics --reqtype gz.msgs.Physics --reptype gz.msgs.Boolean --timeout 2000 --req 'gravity { x: 0 y: 0 z: -2.8 }'

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
• Was GenAI used to generate this PR? If so, make sure to add "Generated-by" to your commits. (See this policy for more info.)

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.

Signed-off-by: Shivang [email protected]

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This is an automatic backport of pull request #3189 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of d2780bb has failed:

On branch mergify/bp/ign-gazebo6/pr-3189
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Unmerged paths:
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	both modified:   src/SimulationRunner.cc
	both modified:   src/systems/physics/Physics.cc
	both modified:   test/integration/user_commands.cc

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