Enhancement: MQTT TLS listeners with multiple TLS versions.

[ioolkos]

The MQTT TLS listeners are currently bound to a specific TLS version. This is per default 'tlsv1.2', as per the following line in the creation of the TLS opts:

vernemq/apps/vmq_server/src/vmq_ssl.erl

Line 111 in a7082c2

{versions, [proplists:get_value(tls_version, Opts, 'tlsv1.2')]}

The TLS protocol version is currently configured per listener, with tls_version. We might want to find a setting for configuring multiple versions, so that we can add them to the Ranch socket opts as {versions, ['tlsv1.2', 'tlsv1.3']}.

In recent Erlang versions, not configuring a version for a TLS server socket will enable tlsv1.2 and tlsv1.3 while tlsv1.1 will be blocked.
We might need to investigate the role of certs and ciphers to configure a combined-versions MQTT listener.