README: Claim oscore-groupcomm-21 conformity

This has not been interop'ed, but it's still better than sticking
with the old version.

README.rst

@@ -45,7 +45,7 @@ This library supports the following standards in full or partially:
   simplistic.
 * RFC8613_ (OSCORE): Full support client-side; protected servers can be
   implemented based on it but are not automatic yet.
-* draft-ietf-core-oscore-groupcomm-17_ (Group OSCORE): Supported for both group
+* draft-ietf-core-oscore-groupcomm-21_ (Group OSCORE): Supported for both group
   and pairwise mode in groups that are fully known. (The lack of an implemented
   joining or persistence mechanism makes this impractical for anything but
   experimentation.)
@@ -65,7 +65,7 @@ the list or in the excluded items, file a wishlist item at the same location).
 .. _RFC8613: https://tools.ietf.org/html/rfc8613
 .. _RFC9176: https://tools.ietf.org/html/rfc9176
 .. _RFC9528: https://tools.ietf.org/html/rfc9528
-.. _draft-ietf-core-oscore-groupcomm-17: https://tools.ietf.org/html/draft-ietf-core-oscore-groupcomm-17
+.. _draft-ietf-core-oscore-groupcomm-21: https://tools.ietf.org/html/draft-ietf-core-oscore-groupcomm-21
 .. _lakers: https://pypi.org/project/lakers-python/
 
 Dependencies