TPA-RFC-83: consider extending the retention limit on mail logs

We currently rotate mail logs daily and keep them for 5 days. That's great for privacy, but not so great for people having to report mail trouble in time. In particular, when there are failures with mail sent just before the weekend, it gives users a very short time frame to report. I propose we extend the retention limit for mail and rspamd logs to 10 days: one week plus flexible friday plus weekend.