Use newer security context instead of identity for LD sig

app/ldsig.py

@@ -41,7 +41,7 @@ def _options_hash(doc: ap.RawObject) -> str:
     for k in ["type", "id", "signatureValue"]:
         if k in doc:
             del doc[k]
-    doc["@context"] = "https://w3id.org/identity/v1"
+    doc["@context"] = "https://w3id.org/security/v1"
     normalized = jsonld.normalize(
         doc, {"algorithm": "URDNA2015", "format": "application/nquads"}
     )