Rename CheckPending to GetVersions

[mfridman]

Picking up from #762 (comment)

I don't want to give the impression this method is suitable for checking "pending" migrations because it's a bit nuanced. I.e., what if a missing (out-of-order) migration has snuck in, it'd be silently ignored if we just compare the max db version and target filesystem version.

This is what HasPending is intended to do, handle all the edge cases around missing migrations (which can be enabled or disabled). For a better understanding of all the possible cases see:

https://github.com/pressly/goose/blob/8c8def43/provider.go#L530-L544 and https://github.com/pressly/goose/blob/8c8def43/internal/gooseutil/resolve_test.go#L73-L83

Once you enumerate the edge cases it becomes clear that internally we cannot compare latest versions, e.g., what happens when running goose up-to 4 and the current db version is 6, where the "missing" migration is 5? I'll write up a reference for "What is a missing migration?"

TL;DR

So, rename CheckPending to GetVersions which does what it says on the tin: get the (max) version from the database and filesystem, which are termed current and target, respectively.