Fix usage of message headers

[chapulina]

🦟 Bug fix

• Related to Don't use ignition/msgs.hh gz-transport#315

Summary

This is needed to compile since gazebosim/gz-transport#315 was merged.

Also fixed a deprecation warning due to gazebosim/gz-sim#1352.

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)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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[codecov]

Codecov Report

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

❗ 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           

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