Port from logrus to log/slog

This doesn't really take much advantage of structured logging beyond what is already done (`id` and `idColor`), and consequently the log handler does not try to do any handling of anything more than that (i.e., grouping, or arbitrary attributes beyond those defined).

One should maybe have a `Context` available to pass in, but there isn't one, and anyway, the log handler doesn't use it, so I've passed in a `TODO` instead.

Everything else is just normal import/rename changes.

Testing Done:
Ran `go run . run`

Reviewed at https://reviews.imfreedom.org/r/2871/