Fix compatibility with protobuf v30 (cpp 6.30.0) - similar to gz-msgs… (backport #677)

[mergify]

Summary

Fix compatibility with protobuf v30 (cpp 6.30.0) - similar to gz-msgs #499

Checklist

• [ x] Signed all commits for DCO
• [ n/a] Added tests
• [ n/a] Updated documentation (as needed)
• [ n/a] Updated migration guide (as needed)
• [ n/a] Consider updating Python bindings (if the library has them)
• [ x] codecheck passed (See contributing)
• [ x] All tests passed (See test coverage)
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[mergify]

Cherry-pick of e1eb227 has failed:

On branch mergify/bp/ign-gui6/pr-677
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You are currently cherry-picking commit e1eb227.
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	both modified:   src/plugins/topic_viewer/TopicViewer.cc

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[scpeters]

There is a test failure on brew, but this fixes the build so I will merge