Change the name of letter_class to postage, this will match the domain language used in the documentation.

Updated the tests to answer review comments.

tests/app/service/test_rest.py

@@ -138,7 +138,7 @@ def test_get_service_by_id(admin_request, sample_service):
     assert 'branding' not in json_resp['data']
     assert json_resp['data']['dvla_organisation'] == '001'
     assert json_resp['data']['prefix_sms'] is True
-    assert json_resp['data']['letter_class'] == 'second'
+    assert json_resp['data']['postage'] == 'second'
 
 
 @pytest.mark.parametrize('detailed', [True, False])