Update Changelog for 13.0.0

[scpeters]

🎈 Changelog for Release

Preparation for 13.0.0 release. The release is not imminent; this is just updating the changelog, which was not done with the earlier prerelease.

Comparison to 12.6.0~pre1: sdformat12_12.6.0-pre1...sdformat13_13.0.0-pre1

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 #1139 (ab2ced7) into sdf13 (02a3de2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            sdf13    #1139   +/-   ##
=======================================
  Coverage   87.00%   87.00%           
=======================================
  Files         125      125           
  Lines       15834    15834           
=======================================
  Hits        13777    13777           
  Misses       2057     2057           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[azeey]

LGTM!

Did you want to include #1065?

[scpeters]

LGTM!

Did you want to include #1065?

That has already been backported to Citadel and Fortress, so I figured it doesn't need to be in this Changelog