update json schema version for observed string extension

extension-definition-specifications/observed-string-8b1/extension-definition--8b1aa84c-5532-4c69-a8e7-b6170facfd3d.json

@@ -1,6 +1,6 @@
 {
     "$id": "https://raw.githubusercontent.com/oasis-open/cti-stix-common-objects/main/extension-definition-specifications/observed-string-8b1/extension-definition--8b1aa84c-5532-4c69-a8e7-b6170facfd3d.json",
-    "$schema": "http://json-schema.org/draft-07/schema#",
+    "$schema": "https://json-schema.org/draft/2020-12/schema",
     "title": "observed-string-extension-schema",
     "description": "This is the wrapper for the observed-strings extension",
     "type": "object",