Removed obsolete RFC from cookie docs.

RFC 2109 was obsoleted by RFC 2965 which was obsoleted by RFC 6265.

docs/ref/request-response.txt

@@ -818,11 +818,11 @@ Methods
 
     .. warning::
 
-        Both :rfc:`2109` and :rfc:`6265` state that user agents should support
-        cookies of at least 4096 bytes. For many browsers this is also the
-        maximum size. Django will not raise an exception if there's an attempt
-        to store a cookie of more than 4096 bytes, but many browsers will not
-        set the cookie correctly.
+        :rfc:`6265` states that user agents should support cookies of at least
+        4096 bytes. For many browsers this is also the maximum size. Django
+        will not raise an exception if there's an attempt to store a cookie of
+        more than 4096 bytes, but many browsers will not set the cookie
+        correctly.
 
 .. method:: HttpResponse.set_signed_cookie(key, value, salt='', max_age=None, expires=None, path='/', domain=None, secure=None, httponly=False, samesite=None)