chore(deps): do not use the experimental package

It's not worth pulling in this dependency just to import trivial constraint definitions. I've copied the code as-is but I don't think uintptr should be there, actually. Shall we remove it?

I was about to add a commit to change gopkg.in/yaml.v3 to go.yaml.in/yaml/v3 but then noticed it is only used in a single place... and that is just some example code (config.NewExtension used in config/examples/basic_usage.go)! It's a bit too much to import such a heavy library just for an example if you ask me. I think it'd be worth moving the example into just docs or a separate Go module.