chore: replace Appveyor with ikalnytskyi/action-setup-postgres - #3966

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I guess this is feature complete.
I guess the main cause of the slowness is the time it takes to establish connections. We could probably add connection caching in the future so parameterized tests reuse connections rather than recreate them for every parameter combination.

@bokken , @sehrope , @davecramer , do you have any comments/suggestions/objections?

I'd prefer to have less external dependencies in the repo over time. I took a peek at ikalnytskyi/action-setup-postgres (never hard of it before...) and it does look pretty clean. Though I don't think we need it.

I'm not sure we even need to build on Windows / MacOS or test against PostgreSQL running on either. If we just had something that could run our already build jar on those against a Linux PostgreSQL server I think it's enough. That'd solve a lot of the startup issues.

Windows tests identified different os behavior regarding sslMode=prefer/allow fallback which look like true os-specific differences to me rather than just ability to build jar on Windows

And this PR touches more than just tests / appveyor. It changes how we handle ssl mode for prefer. You should call that in the title and description.

I've extracted sslMode=allow handling to #3973