Drop version from docker-compose.yml, because it is obsolete (#3184)

Co-authored-by: Eemeli Aro <eemeli@gmail.com>

docker-compose.yml

@@ -1,7 +1,6 @@
 # docker-compose for Pontoon development.
 #
-# Note: Requires docker-compose 1.10+.
-version: '2.3'
+# Note: Requires docker-compose 1.27+.
 services:
   server:
     build: