Update stable identifier version to use a date

Author: Jorgen-5

src/datadoc/datadoc-json-schema.json

@@ -290,6 +290,7 @@
               },
               "stable_identifier_version": {
                 "type": "string",
+                "format": "date",
                 "title": "Stable identifier version",
                 "description": "Version of stable identifier the variable was mapped to prior to pseudonymization.",
                 "$comment": "Only relevant when the variable is mapped to a stable identifier."