Prepare for 8.7.0 Release

[azeey]

🎈 Release

Preparation for 8.7.0 release.

Comparison to 8.6.0: https://github.com/gazebosim/gz-sim/compare/gz-sim8_8.6.0...gz-sim8

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.

[azeey]

Python tests are expected to fail on Homebrew, so I'll go ahead and merge.