Prepare for 6.3.0 release

[mjcarroll]

🎈 6.3.0

Preparation for 6.3.0 release.

Comparison to 6.2.0: gz-physics6_6.2.0...gz-physics6

Needed by gazebosim/gz-sim#1592

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

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 #474 (24fc91e) into gz-physics6 (65a0beb) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           gz-physics6     #474   +/-   ##
============================================
  Coverage        75.52%   75.52%           
============================================
  Files              142      142           
  Lines             7138     7138           
============================================
  Hits              5391     5391           
  Misses            1747     1747           

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[mabelzhang]

On green CI