Update letsencrypt gem

Noticed from omnibus-gitlab#4900 (closed)

Let's Encrypt introduced the breaking change into the API we using. It's currently active only on staging and will be active on production (Nov 1'st 2020)[https://community.letsencrypt.org/t/acme-v2-scheduled-deprecation-of-unauthenticated-resource-gets/74380].

I'm not updating it right away since verification that everything works properly requires some time(api requests are mostly stubbed in specs).