Prefer use of "Postgres" over "PostgreSQL" in docs

[brandur]

I just noticed while reading our Rust crate's README that the use of
"PostgreSQL" in documentation is a dead to rights giveaway that
everything is LLM-written. There's no correct form of "Postgres" versus
"PostgreSQL", but most people in the know (so to speak) tend to write
just "Postgres" and IMO it reads more fluently, though of course this is
subjective.

Here, go through all our public-facing docs and descriptions and convert
uses of "PostgreSQL" to "Postgres".

[bgentry]

LGTM, maybe this is worth a line in AGENTS.md to steer agents this direction?

[brandur]

@bgentry Thx! I threw a terminology line into AGENTS.md.