Summary: extend the retention limit for mail logs to 10 days

# Background

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 issues.

# Proposal

Extend the retention limit for mail (`postfix` and `rspamd`) logs to 10 days: one week, plus "flexible Friday", plus "weekend".

## Goals

Being able to debug mail issues when users notice and/or report them after five days.

## Tasks

Adjust logrotate configuration for syslog-ng and rspamd.

## Scope

All TPA servers.

## Affected users

Sysadmins and email users, which is pretty much everyone.

## Timeline

Logging policies will be changed on Wednesday March 19th.

# References

TPA has various log policies for various services, which we have been meaning to document for a while. This policy proposal doesn't cover for that, see [tpo/tpa/team#40960][] for followup on that more general task.

[tpo/tpa/team#40960]: https://gitlab.torproject.org/tpo/tpa/team/-/issues/40960

See also the [discussion issue][].

[discussion issue]: https://gitlab.torproject.org/tpo/tpa/team/-/issues/42081