DEV: Deprecate array custom fields (discourse#24492)

Array custom fields use separate rows for each value, but whenever we
update an array, we have always destroy the existing rows and create new
ones. Therefore, there's no benefit over using the json type.

app/models/concerns/has_custom_fields.rb

@@ -87,6 +87,13 @@ def append_custom_field(target, key, value)
     end
 
     def register_custom_field_type(name, type)
+      if Array === type
+        Discourse.deprecate(
+          "Array types for custom fields are deprecated, use type :json instead",
+          drop_from: "3.3.0",
+        )
+      end
+
       @custom_field_types ||= {}
       @custom_field_types[name] = type
     end