PostgreSQL 16 release announcement draft

Hi,

Attached is the first draft of the PostgreSQL 16 release announcement,
authored by Chelsea Dole & myself.

To frame this up, the goal of the GA release announcement is to help
folks discover the awesome new features of PostgreSQL. It's impossible
to list out every single feature in the release and still have a
coherent announcement, so we try to target features that have the
broadest range of impact.

It's possible we missed or incorrectly stated something, so please
provide feedback if we did so.

(Note I have not added in all of the links etc. to the Markdown yet, as
I want to wait for the first pass of feedback to come through).

**Please provide feedback by August 26, 12:00 UTC**. After that point,
we need to freeze all changes so we can begin the release announcement
translation effort.

Thanks,

Jonathan