Prepare for 10.3.1 release

[caguero]

🎈 Release

Preparation for 10.3.1 release.

Comparison to 10.3.0: gz-msgs10_10.3.0...gz-msgs10

Checklist

• Asked team if this is a good time for a release
• There are no changes to be ported from the previous major version
• No PRs targeted at this major version are close to getting in
• Bumped minor for new features, patch for bug fixes
• 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.21%. Comparing base (1939fb0) to head (e2e7d1e).
Report is 16 commits behind head on gz-msgs10.

Additional details and impacted files

@@              Coverage Diff              @@
##           gz-msgs10     #475      +/-   ##
=============================================
+ Coverage      98.19%   98.21%   +0.01%     
=============================================
  Files             27       27              
  Lines           1166     1174       +8     
=============================================
+ Hits            1145     1153       +8     
  Misses            21       21              

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