Use slog in place of waterlog

[livingsilver94]

slog is the new structured logging library from the Go project itself. It is available as an experimental package for Go 1.20 and was upstreamed in Go 1.21. This makes it a perfect candidate for our tooling, because:

• it won't bit-rot
• we can use a human-appealing Handler for CLI, and e.g. a JSON Handler on our builders, without changing the code

This PR only replaces waterlog with slog, using its TextHandler Handler. TextHandler is nowhere near the look of waterlog, but a custom Handler mimicking the old look is in the works and will come with a different PR in the near future.

[livingsilver94]

@silkeh would you mind rebasing #13 after this PR is accepted so that we don't keep a miles-long backlog? :) Ideally I would like to see usysconf back amongst the living in two weeks.

[EbonJaeger]

LGTM, though the changes in config.load.go look like a bit more than just changing out the logger. Change scope is something to watch out for in the future, because it makes it harder to review and harder to debug later to figure out what changed caused an issue.

[livingsilver94]

I agree with you, but the code was way more duplicated than I could tolerate 😬 I can split the commit anyway.

[silkeh]

Great work! I have a few relatively minor comments, but it looks fine otherwise 🙂

[livingsilver94]

All sorted, I also squashed the commits.

[silkeh]

LGTM, thanks!