Prepare for 13.9.0 Release

[azeey]

🎈 Release

Preparation for 13.9.0 release. This will be the final release of sdf13 as it has reached EOL with Garden

Part of gazebo-tooling/release-tools#1165

Comparison to 13.8.0: sdformat13_13.8.0...sdf13

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 87.57%. Comparing base (edd5d0b) to head (b48bfc6).
Report is 19 commits behind head on sdf13.

Additional details and impacted files

@@            Coverage Diff             @@
##            sdf13    #1498      +/-   ##
==========================================
- Coverage   87.58%   87.57%   -0.01%     
==========================================
  Files         128      128              
  Lines       17095    17413     +318     
==========================================
+ Hits        14972    15249     +277     
- Misses       2123     2164      +41     

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