Update changelogs for sim 7.0.0

[mjcarroll]

🎈 Release

Preparation for <7.0.0> release.

Comparison to <x.y.z>: ignition-gazebo6_6.12.0...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

Merging #1729 (4242dce) into gz-sim7 (59b76c2) will decrease coverage by 0.02%.
The diff coverage is n/a.

❗ Current head 4242dce differs from pull request most recent head fbd2a72. Consider uploading reports for the commit fbd2a72 to get more accurate results

@@             Coverage Diff             @@
##           gz-sim7    #1729      +/-   ##
===========================================
- Coverage    63.93%   63.91%   -0.03%     
===========================================
  Files          334      334              
  Lines        26333    26333              
===========================================
- Hits         16836    16830       -6     
- Misses        9497     9503       +6     

Impacted Files	Coverage Δ
src/SimulationRunner.cc	91.32% <0.00%> (-0.95%)	⬇️

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