Prepare for 8.4.0 release

[iche033]

🎈 Release

Preparation for 8.4.0 release.

Comparison to 8.3.0: https://github.com/gazebosim/gz-sim/compare/gz-sim8_8.3.0...prep_8.4.0

Checklist

• [x] Asked team if this is a good time for a release
• [x] There are no changes to be ported from the previous major version
• [x] No PRs targeted at this major version are close to getting in
• [x] Bumped minor for new features, patch for bug fixes
• [x] Updated changelog
• [ ] Updated migration guide (as needed)
• [ ] Link to PR updating dependency versions in appropriate repository in gazebo-release (as needed):