Upgrade openmaptiles container to 7.0

Openmaptiles has released the openmaptiles postgis container 7.0 (noticed this while deploying a portal for adfc-hessen). 

I'm currently testing it - I guess we should upgrade as postgres 9 (used in openmaptiles 6) is EOL.

With the upgrade we should also put together a document for people on how to upgrade their existing instance.