Changed "versionadded" after the decision to backport.

Refs #19076.

docs/ref/class-based-views/mixins-simple.txt

@@ -66,7 +66,7 @@ TemplateResponseMixin
 
     .. attribute:: content_type
 
-        .. versionadded:: 1.6
+        .. versionadded:: 1.5
             The ``content_type`` attribute was added.
 
         The content type to use for the response. ``content_type`` is passed