Migrate to slog

[MarvinJWendt]

We currently use zap in internal/log.
Since Go v1.21 slog was introduced to the Go stdlib, and it aims to replace all external logging libraries. We should adapt the template and remove the internal/log package, as the users do not need it anymore (and should prefer to use slog).

Up for discussion

We can discuss if we want to initialize slog in the template, or if we want to let the user do that.