Script can not detect when newer PostgreSQL is used

[krutten]

The pre-start script assumes PostgreSQL 11.6 is the target
https://github.com/rkoster/migrate-postgres-boshrelease/blob/master/jobs/migrate-postgres/templates/pre-start.erb#L96

This is true for cloudfoundry/postgres-release v40, but v41 is 11.7, and newer releases bump PSQL also. We need to bump this to 11.7 for the current cf kit 2.0.0 but if possible, detect the installed target version to make it more independent to release bumps