Add support for PostgreSQL 18

[mprimeaux]

PostgreSQL 18 was released.

It appears explicit support is required by the PostgreSQL operator based on my attempt to upgrade from PostgreSQL 17 to 18.

The postgresql "core-db-1" is invalid: spec.postgresql.version: Unsupported value: "18": supported values: "13", "14", "15", "16", "17"

I'm happy to contribute as required for this to be supported. I'm using version 1.14.0 of the Helm chart.

[FxKu]

First we need to support Postgres 18 in Spilo, then we can also reflect it in the operator.