Pin Elasticsearch client library to 7.13.0

Newer versions of the Elasticsearch Python client library throw an
UnsupportedProductError when used with the "oss" build following
Elastic's licensing changes. This commit pins the client library
to elasticsearch==7.13.0 to avoid this issue.

Author: tw4l

requirements/base.txt

@@ -6,6 +6,7 @@ django-modeltranslation==0.16.1
 django-npm==1.0.0
 djangorestframework==3.12.2
 django-widget-tweaks==1.4.8
+elasticsearch==7.13.0
 elasticsearch-dsl==7.3.0 # pyup: <8.0
 envparse==0.2.0
 gevent==21.1.2