Remove obsolute "version:" from Docker Compose files.

docs/docker-compose.yaml

@@ -1,4 +1,3 @@
-version: '3'
 services:
   docs:
     image: borgmatic-docs

tests/end-to-end/docker-compose.yaml

@@ -1,4 +1,3 @@
-version: '3'
 services:
   postgresql:
     image: docker.io/postgres:13.1-alpine