blogpost: Postgres version update

Created by: efritz

This is a rough initial draft of an announcement on why we're requiring the upgrade from Postgres 9.6 to Postgres 12. This was asked internally and I thought I'd write up thoughts in this format.

This draft is pretty tough and ends kind of abruptly (any advice on how to tidy up the story would be greatly appreciated).

Fixes https://github.com/sourcegraph/sourcegraph/issues/19359.