Prepare for 6.15.1 Release

[azeey]

🎈 Release

Preparation for 6.15.1 release.

Comparison to 6.15.0: ignition-math6_6.15.0...ign-math6

Needed by <PR(s)>

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 ✅

Comparison is base (9a5ad69) 99.38% compared to head (9e62b72) 99.38%.
Report is 1 commits behind head on ign-math6.

Additional details and impacted files

@@            Coverage Diff             @@
##           ign-math6     #572   +/-   ##
==========================================
  Coverage      99.38%   99.38%           
==========================================
  Files             75       75           
  Lines           7029     7029           
==========================================
  Hits            6986     6986           
  Misses            43       43           

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