Test PostgreSQL upgrade

[dirkmueller]

Similarly to #504 we should test upgrades from PostgreSQL x to PostgreSQL x+1

[grisu48]

Is there even an upgrade process defined? The documentation states that "The traditional method for moving data to a new major version is to dump and restore the database [...]".

There is the possibility to use pg_upgrade but last time I tried that required the binaries from the previous and the most recent versions available on the same system/container, which we don't package.

So either we have a good documentation on how to mount/bind/include the binaries from PostgreSQL x into the container of PostgreSQL x+1, or we need to build a custom migration container that contains both binary versions.