PostgreSQL driver updated

[SpiGAndromeda]

Confluence ships with the PostgreSQL driver version 42.2.16.

This could cause problem with some PostgreSQL image versions: FATAL: unsupported frontend protocol 1234.5680: server supports 2.0 to 3.0. According to CONFSERVER-60515 this (and other connection problems) could be solved by using a more up-to-date driver version (e.g. 42.2.18).

With this PR an environment varaible is added to set the driver version. This driver is then downloaded and placed into the WEB-INF/lib directory (other PostgreSQL drivers are removed).

Image building is tested and successful.