Prepare for 6.0.0 Release

[azeey]

🎈 Release

Preparation for 6.0.0 release.

Comparison to 6.0.0-pre2: https://github.com/gazebosim/gz-common/compare/gz-common6_6.0.0-pre2...gz-common6

Needed by <PR(s)>

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]

The windows failure seems flaky. I've created https://github.com/gazebosim/gz-common/issues/644. I'll go ahead and merge this.