Remove version checks

This pull request removes references to psql version checks. It then replaces the PG_ version check macros with FDB_ instead. Everything compiles.

Note that it's not tested yet since the test framework is getting set up in parallel, but this is the first step to pruning some of the parts we won't need.

Generally, the changes fell in these categories:
- Branching behavior based on Postgres version: In most cases, I left it as `if (TRUE)` and put a `// TODO: PG_VERSION_GT(##)` comment there. I wanted to at least keep track of the fact that it was under a version conditional at one point. Either way, we're going to have to go through all of these and remove the ones for unsupported features later, but it might be useful to keep around the `else` branch of the code, which tells us how the driver should work if the feature isn't supported (say, in an older version)
- Checking only if a Postgres version was `>=`: Since this includes the recent versions, I usually opted for the conditionals to take this branch (for now), again marking with a TODO as needed
- Checking only if a Postgres version was `<=`: usually deleted these, though not always
- Setting a configuration option based on version: for many (but not all), I checked for compatibility with the server; for example, `atttypmod` had a lot of branching related to it, but we support it