Prepare for 6.6.0 release

[iche033]

🎈 Release

Preparation for 6.6.0 release.

Comparison to 6.5.0: ignition-gazebo6_6.5.0...prep_6.6.0

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 ignition-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 #1365 (42f9e42) into ign-gazebo6 (d3a5015) will not change coverage.
The diff coverage is n/a.

❗ Current head 42f9e42 differs from pull request most recent head 2d1e4fe. Consider uploading reports for the commit 2d1e4fe to get more accurate results

@@             Coverage Diff              @@
##           ign-gazebo6    #1365   +/-   ##
============================================
  Coverage        62.93%   62.93%           
============================================
  Files              299      299           
  Lines            24167    24167           
============================================
  Hits             15210    15210           
  Misses            8957     8957           

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