Pin elasticsearch to <8.0.0

The latest release 8.0.0 of elasticsearch breaks Zuul when using elasticsearch connection

Trace:
...
File "/usr/local/lib/python3.9/dist-packages/zuul/driver/elasticsearch/con>
self.es = Elasticsearch(
TypeError: __init__() got an unexpected keyword argument 'use_ssl'

Change-Id: Idd4cf9d67f60374fa1e8b9a816845829de31e9f1

requirements.txt

@@ -34,6 +34,6 @@ routes
 jsonpath-rw
 urllib3!=1.25.4,!=1.25.5  # https://github.com/urllib3/urllib3/pull/1684
 cheroot!=8.1.*,!=8.2.*,!=8.3.0 # https://github.com/cherrypy/cheroot/issues/263
-elasticsearch
+elasticsearch<8.0.0
 PyMySQL
 psycopg2-binary