Fix compatibility with protobuf v30

[iche033]

🦟 Bug fix

Summary

Ports the fix from gazebosim/gz-launch#295

This should fix the recent homebrew build failures:

/Users/jenkins/jenkins-agent/workspace/gz_sim-ci-pr_any-homebrew-amd64/gz-sim/src/systems/websocket_server/WebsocketServer.cc:1053:24: error: invalid operands to binary expression ('basic_string<char, char_traits<char>, allocator<char>>' and 'absl::string_view' (aka 'basic_string_view<char>'))

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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