Move Postgres to its own network for better isolation

A lot of services are yet to be updated to start connecting to
`devture_postgres_container_network` as an additional network.
Many are already done, but I'll go through all the others later.

group_vars/matrix_servers

@@ -3178,8 +3178,6 @@ devture_postgres_architecture: "{{ matrix_architecture }}"
 
 devture_postgres_base_path: "{{ matrix_base_data_path }}/postgres"
 
-devture_postgres_container_network: "{{ matrix_docker_network }}"
-
 devture_postgres_uid: "{{ matrix_user_uid }}"
 devture_postgres_gid: "{{ matrix_user_gid }}"