This is a breaking change. The certificate indicated by 'CAfile' is no
longer used as is in 'certificate-chain' (along with the leaf cert).
The chain returned by the ACME v2 endpoint is used instead. This allows
for more flexbility with respect to key/CA rotation, cf.
https://letsencrypt.org/2020/11/06/own-two-feet.html and
https://community.letsencrypt.org/t/beginning-issuance-from-r3/139018
Moreover 'CAfile' now defaults to @@datadir@@/lacme/ca-certificates.crt
which is a concatenation of all known active CA certificates (which
includes the previous default).