Prepare for 6.8.0 Release

[azeey]

🎈 Release

Preparation for 6.8.0 release.

Comparison to 6.7.1: ignition-sensors6_6.7.1...ign-sensors6

Needed by ros-navigation/navigation2#3634 (comment)

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.