Update Changelog, README and prepare for gz-common6.0.0~pre1 release

[iche033]

🎈 Release

Preparation for 6.0.0~pre1 release.

Comparison to 5.6.0: gz-common5_5.6.0...ionic_changelog

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.