Add ros_gz_bridge as dependency to demos (backport #256)

[mergify]

While building from source, rosdep does not install ros_gz_bridge otherwise.

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This is an automatic backport of pull request #256 done by Mergify.

[mergify]

Cherry-pick of b35100d has failed:

On branch mergify/bp/humble/pr-256
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit b35100d.
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Unmerged paths:
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	both modified:   ign_ros2_control_demos/package.xml

no changes added to commit (use "git add" and/or "git commit -a")

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