Prepare for 6.4.0 Release

[azeey]

🎈 Release

Preparation for 6.4.0 release.

Comparison to 6.3.2: gz-physics6_6.3.2...gz-physics6

Needed by gazebosim/gz-sim#1949

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 #512 (e72c864) into gz-physics6 (c80d4d7) will increase coverage by 0.01%.
The diff coverage is 50.00%.

❗ Current head e72c864 differs from pull request most recent head 826bb25. Consider uploading reports for the commit 826bb25 to get more accurate results

@@               Coverage Diff               @@
##           gz-physics6     #512      +/-   ##
===============================================
+ Coverage        76.16%   76.18%   +0.01%     
===============================================
  Files              142      142              
  Lines             7251     7251              
===============================================
+ Hits              5523     5524       +1     
+ Misses            1728     1727       -1     

Impacted Files	Coverage Δ
dartsim/src/SDFFeatures.cc	64.92% <50.00%> (+0.20%)	⬆️