Doc: Clean up references to obsolete OS versions.

Remove obsolete instructions for old operating system versions, and
update the text to reflect the defaults on modern systems.

Reviewed-by: Tom Lane <[email protected]>
Reviewed-by: Peter Eisentraut <[email protected]>
Reviewed-by: Magnus Hagander <[email protected]>
Discussion: https://postgr.es/m/CA%2BhUKGLmJUSwybaPQv39rB8ABpqJq84im2UjZvyUY4feYhpWMw%40mail.gmail.com

Author: macdice