Replace log with slog

[aratz-lasa]

What 🕵️‍♀️

This PR replaces log with slog, a structured logger that will soon become part of the standard Go library.

Why 🔑

It makes logging more performant and tunable.

Testing 🧪

• [x] go test ./... from root
• [x] go run ./examples/dnsregistry/main.go from root

Demo

[aratz-lasa]

@richardartoul slog has an option for adding the source code position on every log. I've never used that because I don't find it necessary. But would you like to activate it or set a flag for activating it?

[richardartoul]

Looks good! Just have some nits.

Thanks for doing this!