Replace base64.encodestring with base64.encodebytes

Base64.encodestring has been deprecated since 3.1 and removed
in python 3.9,Replace it with base64.encodebytes from python3.1[1].

[1]https://docs.python.org/3.9/library/base64.html?highlight=deprecated#base64.encodebytes

Change-Id: I159bf1db5e74c5c5e604d4f11660c2a0be916ae0

plugins/modules/memcached

@@ -302,7 +302,7 @@ class Memcached(object):
         """
 
         try:
-            b64_value = base64.encodestring(value)
+            b64_value = base64.encodebytes(value)
         except Exception as exp:
             self._failure(
                 error=str(exp),