Prepare for 9.4.0 release

[iche033]

🎈 Release

Preparation for 9.4.0 release.

Comparison to 9.3.0: https://github.com/gazebosim/gz-msgs/compare/gz-msgs9_9.3.0...prep_9.4.0

Needed by https://github.com/gazebosim/gz-sim/pull/1898

Checklist

• [x] Asked team if this is a good time for a release
• [x] There are no changes to be ported from the previous major version
• [x] No PRs targeted at this major version are close to getting in
• [x] Bumped minor for new features, patch for bug fixes
• [x] Updated changelog
• [ ] Updated migration guide (as needed)
• [ ] Link to PR updating dependency versions in appropriate repository in gazebo-release (as needed):

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 #329 (71b34a5) into gz-msgs9 (2aaece3) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 71b34a5 differs from pull request most recent head 69b051a. Consider uploading reports for the commit 69b051a to get more accurate results

@@            Coverage Diff            @@
##           gz-msgs9     #329   +/-   ##
=========================================
  Coverage     95.41%   95.41%           
=========================================
  Files            10       10           
  Lines          1026     1026           
=========================================
  Hits            979      979           
  Misses           47       47           

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