Prepare for 8.4.0 release

[caguero]

Prepare for 8.4.0 Release

🎈 Release

Preparation for 8.4.0 release.

Comparison to 8.3.0: gz-math8_8.3.0...gz-math8

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 95.62%. Comparing base (44e3bba) to head (9d25ec3).
⚠️ Report is 50 commits behind head on gz-math8.

Additional details and impacted files

@@             Coverage Diff              @@
##           gz-math8     #823      +/-   ##
============================================
- Coverage     95.71%   95.62%   -0.10%     
============================================
  Files           150      158       +8     
  Lines         10390    11042     +652     
============================================
+ Hits           9945    10559     +614     
- Misses          445      483      +38     

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