Reimplement #authenticate using #auth

IMO, the method signature for `#authenticate` is confusing and it the
method should be deprecated in favor of `#auth`.

lib/net/smtp.rb

@@ -896,18 +896,15 @@ def open_message_stream(from_addr, *to_addrs, &block)   # :yield: stream
     # All arguments—other than +type+—are forwarded to the authenticator.
     # Different authenticators may interpret the +username+ and +secret+
     # arguments differently.
-    def authenticate(*args, type: nil, **kwargs, &block)
+    def authenticate(*args, **kwargs, &block)
       case args.length
       when 1, 3 then typearg = args.pop
       when (4..)
         raise ArgumentError, "wrong number of arguments " \
                              "(given %d, expected 0..3)" % [args.length]
       end
-      authtype = type || typearg || DEFAULT_AUTH_TYPE
-      check_auth_method authtype
       check_auth_args(*args) unless args.empty?
-      authenticator = Authenticator.auth_class(authtype).new(self)
-      critical { authenticator.auth(*args, **kwargs) }
+      auth(typearg, *args, **kwargs, &block)
     end
 
     # call-seq: