Prepare for 8.1.0 Release

[azeey]

🎈 Release

Preparation for 8.1.0 release.

Comparison to 8.0.0: gz-sim8_8.0.0...gz-sim8

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 (448463a) 65.97% compared to head (32c23ed) 65.95%.

Additional details and impacted files

@@             Coverage Diff             @@
##           gz-sim8    #2308      +/-   ##
===========================================
- Coverage    65.97%   65.95%   -0.02%     
===========================================
  Files          327      327              
  Lines        31329    31329              
===========================================
- Hits         20668    20662       -6     
- Misses       10661    10667       +6     

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