Replace Protobuf DebugString with PrintToString (backport #703)

[mergify]

🦟 Bug fix

Summary

As of Protobuf 30, DebugString may contain some randomized content in order to redact sensitive information. This adapts gazebosim/gz-transport#685 for gz-gui.

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)
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This is an automatic backport of pull request #703 done by Mergify.

[mergify]

Cherry-pick of 0262982 has failed:

On branch mergify/bp/ign-gui6/pr-703
Your branch is up to date with 'origin/ign-gui6'.

You are currently cherry-picking commit 0262982.
  (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/plugins/publisher/Publisher.cc
	both modified:   src/plugins/topic_echo/TopicEcho.cc

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