Update extension to use APIv4 and EntityRefs
This PR updates the automatchinggift extension to use CiviCRM's APIv4 instead of APIv3, as well as changes Contact References to Entity References.
The 'Matching Gift Organizations' Entity Ref makes use of the Advanced Filter on the custom field to limit the contacts who can be selected for this field to those who are added to the group named 'Matching_Gift_Organizations'.