upgrade certificate fingerprint hash
The "SSL Certificate check" message currently displays certificate fingerprints in MD5 and SHA1:
- https://gitlab.com/muttmua/mutt/blob/master/mutt_ssl.c#L1203
- https://gitlab.com/muttmua/mutt/blob/master/mutt_ssl_gnutls.c#L957
MD5 is basically dead, it has been since 2011: https://tools.ietf.org/html/rfc6151
SHA-1 should be avoided, especially since https://shattered.io/
I propose to remove MD5 and add SHA-256/SHA-512, keeping SHA-1 for the time being.