update _examples/logging to use new "slog"

[pkieltyka]

Go 1.20 will ship with new "slog" package. We can use it now via "golang.org/x/exp/slog"

Let's upgrade https://github.com/go-chi/chi/blob/master/_examples/logging/main.go to use it instead

[ydarb]

@pkieltyka Please assign me to this, I've created a PR for it

[pkieltyka]

thank you to @ydarb for their awesome PR and contribution to close out this issue