Make the default duration more aggressive to prepare for upcoming CA/B forum re-verification updates

[treidel]

Proposed changes to the CA/B forum reverification requirements will require domains to be re-verified more often than currently is required. This ticket requests the default period to be chosen in alignment with those anticipated changes, e.g. possibly 60 days or even 20 days.

[vanbroup]

Currently domains will be renewed 31 days before expiry (see the link below) unless a different preference is set by the suer, I think this should be sufficient to cover the proposed 90-day validity.

https://github.com/EntrustCorporation/dv/blob/main/main.go#L32

If the validity becomes sixty or even thirty days, we might need to move to a strategy where we renew date becomes a percentage of the validity, such as 50% of 30 days would be every 15 days, etc.

Do you think that 31 days before expiry is currently sufficient?