Remove outdated versionadded 5.* directives

For version 6.0, no versionadded <6.0 are allowed.

Author: MarkPedron

http_client.rst

@@ -1688,10 +1688,6 @@ responses dynamically when it's called::
         $client = new MockHttpClient();
         $client->setResponseFactory($responses);
 
-    .. versionadded:: 5.4
-
-        The ``setResponseFactory()`` method was introduced in Symfony 5.4.
-
 If you need to test responses with HTTP status codes different than 200,
 define the ``http_code`` option::
 

serializer.rst

@@ -149,11 +149,6 @@ configuration:
             ;
         };
 
-.. versionadded:: 5.4
-
-    The ability to configure the ``default_context`` option in the
-    Serializer was introduced in Symfony 5.4.
-
 .. _serializer-using-serialization-groups-annotations:
 
 Using Serialization Groups Annotations