Update gnupg to 2.2.4. Bump the libassuan requirement to 2.5.1.

• gpg: Change default preferences to prefer SHA512.
• gpg: Print a warning when more than 150 MiB are encrypted using a cipher with 64 bit block size.
• gpg: Print a warning if the MDC feature has not been used for a message.
• gpg: Fix regular expression of domain addresses in trust signatures. [#2923]
• agent: New option --auto-expand-secmem to help with high numbers of concurrent connections. Requires libgcrypt 1.8.2 for having an effect. [#3530]
• dirmngr: Cache responses of WKD queries.
• gpgconf: Add option --status-fd.
• wks: Add commands --check and --remove-key to gpg-wks-server.
• Increase the backlog parameter of the daemons to 64 and add option --listen-backlog.
• New configure option --enable-run-gnupg-user-socket to first try a socket directory which is not removed by systemd at session end.