Replace go-kit/log with log/slog

[yeya24]

Similar to prometheus/prometheus#14355

Prometheus has switched to use log/slog and remove go-kit/log. In order to be compatible with Prometheus to pull latest Prometheus code, Thanos needs to switch to use slog.

This can be used as a meta issue to split the work and track the progress.

Related:

• pkg/reloader: Provide option to use slog instead of go-kit #7677

[yeya24]

It might be too much work to replace the whole go-kit/log to use slog. It might be good to go with the approach of adapter using https://github.com/tjhop/slog-gokit