Prepare for 8.1.0 release

[mjcarroll]

🎈 Release

Preparation for 8.1.0 release.

Comparison to 8.0.0: gz-math8_8.0.0...gz-math8

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 ✅

Project coverage is 95.71%. Comparing base (44e3bba) to head (3c52f5f).
Report is 4 commits behind head on gz-math8.

Additional details and impacted files

@@            Coverage Diff            @@
##           gz-math8     #650   +/-   ##
=========================================
  Coverage     95.71%   95.71%           
=========================================
  Files           150      150           
  Lines         10390    10390           
=========================================
  Hits           9945     9945           
  Misses          445      445           

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