ops: pin docker-compose version to 3.4

The `docker-compose` setup uses features that
are not available in all versions. This pins
the `docker-compose` setup to version `3.4`
instead of just `3`.

It is known to work under the following versions:

```
$ uname -or
5.10.70-1-MANJARO GNU/Linux

$ docker-compose --version
Docker Compose version 2.0.1

$ docker --version
Docker version 20.10.9, build c2ea9bc90b
```

ops/docker-compose-metrics.yml

-version: "3"
+version: "3.4"
 
 services:
   l2geth:
@@ -51,4 +51,4 @@ volumes:
   influxdb_data:
   grafana_data:
   grafana_dashboards:
-  prometheus_data:
+  prometheus_data:
\ No newline at end of file

ops/docker-compose-nobuild.yml

-version: "3"
+version: "3.4"
 services:
   l1_chain:
     image: ethereumoptimism/hardhat-node:${DOCKER_TAG:-prerelease-0.5.0-rc-7-ee217ce}

ops/docker-compose-rpc-proxy.yml

-version: "3"
+version: "3.4"
 services:
   rpc-proxy:
     depends_on:

ops/docker-compose.yml

-version: "3"
+version: "3.4"
 
 services:
   # this is a helper service used because there's no official hardhat image