The log level for the apps is using `log/slog` from Go standard library.
This commit change log format for fiber http logger to match with
the slog standard log format (date and time).
This commit also remove `APP_DEBUG` field from config struct.
TODO:
Use `slog` for default app output. Note that in this commit, the `slog`
output only implemented in `cron` "db migrate" and probe client.
