Updated Docker Compose files to be compatible with version 3

docker-compose-host.yml

-version: "2.1"
+version: "3"
 
 services:
   credentials:

docker-compose-marketing-site-host.yml

-version: "2.1"
+version: "3"
 
 services:
   marketing:

docker-compose-marketing-site-sync.yml

-version: "2.1"
+version: "3"
 
 services:
   marketing:

docker-compose-marketing-site.yml

-version: "2.1"
+version: "3"
 
 services:
   lms:

docker-compose-sync.yml

-version: "2.1"
+version: "3"
 
 services:
   credentials:

docker-compose.yml

@@ -8,7 +8,7 @@
 # - Every service's container name should be prefixed with "edx.devstack." to avoid conflicts with other containers
 #   that might be running for the same service.
 
-version: "2.1"
+version: "3"
 
 services:
   # Third-party services