Unable to disable TLSv1.3 or fallback to TLSv1.2 when 1 cipher is disabled

[Steve Egbert]

>> I have an operational need to disable TLSv1.3 due to inadequate support to exclude certain ciphers.
> 
> There is no need to disable TLSv1.3 and attempts to do so will be flagged as “downgrade attacks”.

Let us ignore TLSv1.2 as a downgrade option. And focus on TLSv1.3 for 
its entirety of this thread.

If the ciphersuite (not cipher for that's a TLSv1.2 term), but a 
ciphersuite for TLSv1.3.... needs to have its set of ciphers:

* Reordered, or
* disabled

We cannot do it at the moment given this snapshot of Dovecot.