contrib: Bump docker-compose version to 3.4

'start_period' for 'healthchecks' was added in Compose file version 3.4,

https://docs.docker.com/compose/compose-file/compose-versioning/#version-34

Bump docker-compose files' versions from 3.3 to 3.4

Without this, docker-compose (1.25 at least) returns

ERROR: The Compose file './contrib/docker-compose/caddy.yml' is invalid because:
services.db.healthcheck value Additional properties are not allowed ('start_period' was unexpected)

contrib/docker-compose/basic.yml

@@ -1,4 +1,4 @@
-version: '3.3'
+version: '3.4'
 services:
   miniflux:
     image: miniflux/miniflux:latest
@@ -32,4 +32,4 @@ services:
       interval: 10s
       start_period: 30s
 volumes:
-  miniflux-db:
+  miniflux-db:

contrib/docker-compose/caddy.yml

@@ -1,4 +1,4 @@
-version: '3.3'
+version: '3.4'
 services:
   caddy:
     image: caddy:2

contrib/docker-compose/traefik.yml

@@ -1,4 +1,4 @@
-version: '3.3'
+version: '3.4'
 services:
   traefik:
     image: "traefik:v2.3"