Fix unused result warnings (backport #570)

[mergify]

🦟 Bug fix

Fixes compiler warnings, related to gazebosim/gz-transport#810

Summary

As noted in the protobuf 34.0 release notes [[nodiscard]] has been added to many APIs, which is causing several unused-result compiler warnings in this package:

• https://build.osrfoundation.org/job/gz_msgs-ci-pr_any-homebrew-arm64/53/ (from Remove major version numbers from install paths #569)

This fixes the warnings in test code in 9005375 and in DynamicFactory in 12f2f82

Checklist

• Signed all commits for DCO
• Added a screen capture or video to the PR description that demonstrates the fix (as needed)
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
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• All tests passed (See test coverage)
• Updated Bazel files (if adding new files). Created an issue otherwise.
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---

This is an automatic backport of pull request #570 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 60b4f77 has failed:

On branch mergify/bp/ign-msgs8/pr-570
Your branch is up to date with 'origin/ign-msgs8'.

You are currently cherry-picking commit 60b4f77.
  (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)

Changes to be committed:
	modified:   test/integration/headers.cc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   core/src/DynamicFactory.cc

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

[scpeters]

this backport fixes 5 of 6 warnings

There is still a warning at Factory.cc:254 that was fixed in gz-msgs10 and newer by #384

[scpeters]

I'll merge for now with these fixes