Add PostgreSQL 18 beta images

[gbartolini]

In preparation for the release of CloudNativePG 1.27, we need to start running E2E tests on PostgreSQL 18.

Please enable system containers for 18 beta (the process should be in place already) and introduce the beta feature process to the bake based building process.

[gbartolini]

@NiccoloFei reminded me that we already have the minimal and standard images for Postgres 18 beta1.

See: https://github.com/cloudnative-pg/postgres-containers/blob/main/docker-bake.hcl#L41