Increase supported macOS version

What does this Merge Request do?

Mavericks has long been out of security coverage by Apple. Bump up to High Sierra which still seems to receive security updates

Merge Request checklist

• [-] Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.