Move from go-kit logger to slog

[pedro-stanaka]

Summary

The whole Prometheus ecosystem is moving away from go-kit Logging library towards Go's stdlib slog package. We should also make the move to be in line with what the ecosystem is using.

See work on Prometheus itself: prometheus/prometheus#14906

Seems like the author of that PR published a helper we can use: github.com/tjhop/slog-gokit