Fix usage of message headers

[chapulina]

🦟 Bug fix

• Related to https://github.com/gazebosim/gz-transport/pull/315

Summary

This is needed to compile since https://github.com/gazebosim/gz-transport/pull/315 was merged.

Also fixed a deprecation warning due to https://github.com/gazebosim/gz-sim/pull/1352.

Checklist

• [x] 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)
• [x] codecheck passed (See contributing)
• [x] All tests passed (See test coverage)
• [x] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸

[codecov[bot]]

Codecov Report

Merging #196 (d264b36) into gz-launch6 (178645f) will not change coverage. The diff coverage is n/a.

:exclamation: Current head d264b36 differs from pull request most recent head 5b807b2. Consider uploading reports for the commit 5b807b2 to get more accurate results

@@             Coverage Diff             @@
##           gz-launch6     #196   +/-   ##
===========================================
  Coverage       56.90%   56.90%           
===========================================
  Files               3        3           
  Lines             413      413           
===========================================
  Hits              235      235           
  Misses            178      178           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.