Prepare for 6.15.0 Release

[azeey]

🎈 Release

Preparation for 6.15.0 release.

Comparison to 6.14.0: ignition-math6_6.14.0...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

Merging #554 (c3f2b63) into ign-math6 (a0c9148) will not change coverage.
The diff coverage is n/a.

❗ Current head c3f2b63 differs from pull request most recent head 2834bef. Consider uploading reports for the commit 2834bef to get more accurate results

@@            Coverage Diff             @@
##           ign-math6     #554   +/-   ##
==========================================
  Coverage      99.38%   99.38%           
==========================================
  Files             75       75           
  Lines           7029     7029           
==========================================
  Hits            6986     6986           
  Misses            43       43