Also handle SIGTERM gracefully (backport #2747)

[mergify]

🦟 Bug fix

Fixes #2738

Summary

As proposed by @azeey, in addition to SIGINT, also catch SIGTERM, which is the default signal sent when trying to stop a process externally.

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

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This is an automatic backport of pull request #2747 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 13bf5be has failed:

On branch mergify/bp/ign-gazebo6/pr-2747
Your branch is up to date with 'origin/ign-gazebo6'.

You are currently cherry-picking commit 13bf5bea.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/cmd/cmdgazebo.rb.in

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

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally