goflow2: use slog instead of logrus

[lspgn]

This removes the logrus dependency as it's in maintenance-only mode. Haven't used slog before and it's pretty new. Looking for feedbacks.

[lspgn]

Base one is less fancy without colors. Native JSON rendering is useful.

{"time":"2023-08-09T21:43:07.940616-07:00","level":"INFO","msg":"starting GoFlow2"}
{"time":"2023-08-09T21:43:07.941039-07:00","level":"INFO","msg":"starting collection","scheme":"sflow","hostname":"","port":6343,"count":1}
{"time":"2023-08-09T21:43:07.941242-07:00","level":"INFO","msg":"starting collection","scheme":"netflow","hostname":"","port":2055,"count":1}

{"time":"2023-08-09T21:43:09.525804-07:00","level":"INFO","msg":"receiver closed","scheme":"sflow","hostname":"","port":6343,"count":1}
{"time":"2023-08-09T21:43:09.525828-07:00","level":"INFO","msg":"transporter closed"}
{"time":"2023-08-09T21:43:09.525854-07:00","level":"INFO","msg":"receiver closed","scheme":"netflow","hostname":"","port":2055,"count":1}
{"time":"2023-08-09T21:43:09.52589-07:00","level":"INFO","msg":"closed HTTP server","http":":8080"}

[gaby]

👍 for adding this, logrus is also super slow