Acme.sh switching to ZeroSSL by default -> should be set to Let's Encrypt

Summary

The acme.sh script will use ZeroSSL as default CA for new certificates starting August 1 2021. See https://community.letsencrypt.org/t/the-acme-sh-will-change-default-ca-to-zerossl-on-august-1st-2021/144052 We should stick to Let's Encrypt, so we need to set this as hard default for acme.sh

Proposed fix

Set the default CA for acme.sh to Let's Encrypt on install and also for existing users. While doing this, maybe we could set acme.sh to be auto updating aswell?

To set the default, this should be run:
acme.sh --set-default-ca --server letsencrypt
To update acme.sh and enable auto updating:
acme.sh --upgrade --auto-upgrade