Update docker-compose files version (#2920)

docker/docker-compose.middleware.yaml

@@ -1,4 +1,4 @@
-version: '3.1'
+version: '3'
 services:
   # The postgres database.
   db:

docker/docker-compose.yaml

@@ -1,4 +1,4 @@
-version: '3.1'
+version: '3'
 services:
   # API service
   api: