Update changelog for 6.0.0

[nkoenig]

🎈 Change log Release

Preparation for 6.0.0 release.

Comparison is broken, probably because I merged incorrectly once. I used this.

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 #201 (c8bccfc) into gz-launch6 (9fe4564) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           gz-launch6     #201   +/-   ##
===========================================
  Coverage       56.90%   56.90%           
===========================================
  Files               3        3           
  Lines             413      413           
===========================================
  Hits              235      235           
  Misses            178      178           

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