Documentation: Link to PostgreSQL JDBC drvier is not correct any more

Hi,

on the Guacamole Database authentication documentation site https://guacamole.apache.org/doc/gug/jdbc-auth.html

the link to the postgreSQL JDBC driver is not correct any more.

current link destination: https://jdbc.postgresql.org/download.html#current

new link destination: https://jdbc.postgresql.org/download/#latest-versions