[TODO]: Deprecate TLS_LEVEL=intermediate

[polarathene]

Description

Despite what the docs presently say for TLS_LEVEL, intermediate does not offer TLS 1.0/1.2 anymore:

docker-mailserver/target/scripts/helpers/ssl.sh

Lines 145 to 148 in a815bf5

	( "intermediate" )
	local TLS_INTERMEDIATE_SUITE='ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:DHE-RSA-AES128-SHA256:DHE-RSA-AES256-SHA256'
	local TLS_INTERMEDIATE_IGNORE='!SSLv2,!SSLv3,!TLSv1,!TLSv1.1'
	local TLS_INTERMEDIATE_MIN='TLSv1.2'

This was removed in #2945 but I have noticed that we have a user forking to carry a patch to revert back the support along with the required OpenSSL config.

---

From v15 (or perhaps v14 if someone wants to tackle it in time), we could add this support back via an alternative TLS_LEVEL=legacy or alternative opt-in like LEGACY_TLS=1.

I'm not sure if there is any value in us maintaining the separate intermediate list, the cipher lists could be unset back to defaults from Postfix/Dovecot. TLS_LEVEL would then be deprecated so that we only offer modern, as it's really only exists for legacy requirements to use intermediate for broader compatibility 🤷‍♂️

We don't presently document to users how to bring back TLS <1.2 support, although we could do take this approach instead with a user-patches.sh if maintainers do not want to continue carrying such support within DMS officially for user convenience.