Simplify PostgreSQL setup in Github Actions

[roji]

Our CI builds currently download various versions of PostgreSQL and install them; this is sometimes the source of flakiness (e.g. when the EnterpriseDB Windows distribution isn't accessible) and is generally complex (e.g. download and parse an XML to find the latest EDB version). At the same time, Github Actions machine images come with a recent version of PostgreSQL pre-installed (currently 14).

We could simplify our CI as follows:

1. Switch the Windows and MacOS builds to use the built-in PostgreSQL.
2. Possibly switch to using Docker to get PostgreSQL on Linux.

This would mean that we continue testing all versions of PG (10-15) on Linux, but only test the latest recent (14) on Windows/MacOS; that seems like good enough coverage (there's unlikely be a bug in some version of PG which only affects non-Linux, etc.). Similarly, PostGIS may end up getting tested on Linux only (not sure if it's pre-installed on Windows/MacOS), which again seems fine.

[roji]

We should do the first part, i.e. switch Windows/Mac to just use the PG version that comes built-in; PG 10 is no longer installable on Windows (we could also just drop it entirely but why not continue testing for Linux). In addition, the Windows installation is slow, sometimes flaky, and testing the multiple versions on Windows doesn't seem like it provides much coverage beyond doing it on Linux.

@vonzshik @Brar @NinoFloris any thoughts on this?

[vonzshik]

We can still install pg 10 on windows, we just have to use a specific version (10.23-1) instead of relying on getting the latest version from postgresql site.

All in all, I'm ok with using the pre installed version of pg on windows/macos.

[NinoFloris]

PG doesn't really have a long history of diverging behavior across OSes.

So indeed let's use linux to test compatibility of all versions with npgsql. The one version on Windows/macOS would just be there as a smoke test for cross platform compliance.

Also, officially PG 10 went out of support since the issue was opened so I wouldn't mind dropping it at all

[NinoFloris]

@roji @vonzshik Are we still OK with dropping the pg version matrix on windows?

[roji]

I personally am...

[vonzshik]

I'm also OK with it.

[roji]

Cool... Now we can consider doing the Linux PG via a docker service container if we want to...

There's also the testcontainer option, but I'm not sure to what extent there's enough extensibility there to allow us to configure PG exactly as we want to..