Update Changelog for 6.0.0

[adityapande-1995]

🎈 Changelog 6.0.0

Preparation for 6.0.0 release.

Comparison to <5.2.0>: ignition-physics5_5.2.0...gz-physics6

Needed by gazebo-tooling/release-tools#578

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):

[codecov]

Codecov Report

Merging #427 (92a094f) into gz-physics6 (856e584) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           gz-physics6     #427   +/-   ##
============================================
  Coverage        73.69%   73.69%           
============================================
  Files              128      128           
  Lines             5787     5787           
============================================
  Hits              4265     4265           
  Misses            1522     1522           

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