dovecot: 2.3.0.1 -> 2.3.1

Semi-automatic update generated by https://github.com/ryantm/nix-update tools.

This update was made based on information from https://repology.org/metapackage/dovecot/versions.

These checks were done:

- built on NixOS
- ran `/nix/store/c20ip7wyymd39l7zisx38ky3bxp1sybv-dovecot-2.3.1/bin/dovecot --help` got 0 exit code
- ran `/nix/store/c20ip7wyymd39l7zisx38ky3bxp1sybv-dovecot-2.3.1/bin/dovecot --version` and found version 2.3.1
- found 2.3.1 with grep in /nix/store/c20ip7wyymd39l7zisx38ky3bxp1sybv-dovecot-2.3.1
- directory tree listing: https://gist.github.com/6d90467ee7649d7efc0a48eeacfc42c8