model: Core: DateTime: Inherit from xsd:dateTimeStamp

Inherit from xsd:dateTimeStamp instead of xsd:string. The XSD definition
is compatible with the SPDX definition, and this will allow code
generation tools to map the value to a code-native datatype.

model/Core/Datatypes/DateTime.md

@@ -15,7 +15,7 @@ The specific format is one of the most commonly used ISO-8601 formats.
 ## Metadata
 
 - name: DateTime
-- SubclassOf: xsd:string
+- SubclassOf: xsd:dateTimeStamp
 
 ## Format