gnu: dovecot: Update to 2.3.0.

* gnu/packages/mail.scm (dovecot): Update to 2.3.0.
* gnu/services/mail.scm (dovecot-configuration)[director-doveadm-port]
[ssl-parameters-regenerate]: Delete fields.
[ssl-protocols]: Rename to...
[ssl-min-protocol]: ...this.
[mail-log-prefix, mdbox-rotate-size, ssl-cipher-list, imap-logout-format]:
Update default values.
* doc/guix.texi (Mail Services): Reflect the above changes to the service.