Prepare for 7.7.0 Release

[azeey]

🎈 Release

Preparation for 7.7.0 release.

Comparison to 7.6.0: gz-sim7_7.6.0...gz-sim7

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 ✅

Comparison is base (c6cd6ed) 64.71% compared to head (0374146) 64.72%.

Additional details and impacted files

@@           Coverage Diff            @@
##           gz-sim7    #2292   +/-   ##
========================================
  Coverage    64.71%   64.72%           
========================================
  Files          357      357           
  Lines        29160    29160           
========================================
+ Hits         18872    18873    +1     
+ Misses       10288    10287    -1     

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