Support Debian 11 (Bullseye)

[mig5]

Hi,

This role didn't work with a Debian version higher than Buster.

I've tweaked the logic a bit to not be a 'stretch or buster' condition but 'it's not jessie'. This of course means that super old distributions, like wheezy, squeeze etc, won't work (they'll evaluate to use the 'newer' certbot option like stretch/buster/bullseye), but I suspect probably didn't already, so it's probably an acceptable risk.

Thanks!