mail/dovecot24: new port

• adding the new version 2.4 of dovecot, while keeping 2.3 in place
• please note that the config syntax has changed between 2.3 and 2.4 and needs to be manually adapted, so use/upgrade with care

PR: 286695
Reported by: Ian Dickens <ian_dickens@icloud.com>
Author: Jordan Ostreff <jordan@ostreff.info>
Co-authored-by: Sebastian Oswald <sko@rostwald.de>
Discussed with: Sebastian Oswald <sko@rostwald.de>,

		Eugene R <genie@geniechka.ru>,
		Herbert J. Skuhra <herbert@gojira.at>,
		Christos Chatzaras <chris@cretaforce.gr>,
		and others

Tested by: Jordan Ostreff <jordan@ostreff.info>