Prepare for 6.1.0 release

[nkoenig]

Signed-off-by: Nate Koenig [email protected]

🎈 Release

Preparation for 6.1.0 release.

Comparison to 6.1.0: ignition-gui6_6.0.0...ign-gui6

Needed by gazebosim/gz-sim#1109

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
• Link to PR updating dependency versions in appropriate repository in ignition-release (as needed):

Note to maintainers: Remember to use Squash-Merge