Set minimum TLS version to 1.2

[link2xt]

No description provided.

[link2xt]

Maybe we need to think about compatibility to older servers, but Thunderbird currently requires version 1.2 and does not seem to have reported problems. Firefox allows anything >= TLS1.0.

[link2xt]

Also related, long-standing feature request on TLS 1.3 support: sfackler/rust-native-tls#140

[r10s]

Maybe we need to think about compatibility to older servers [...]

nauta comes to my mind, @adbenitez do you know sth. about if nauta support TLS 1.2? however, maybe we should merge that just in, we will probably find out before the next official release by testing versions.

if needed, we can think over lowering the requirements if "strict tls" is disabled by the user/the server.

[link2xt]

if needed, we can think over lowering the requirements if "strict tls" is disabled by the user/the server.

I thought about doing it. It has a bad effect that if the server has a valid certificate and only supports TLS 1.1, we will switch to strict_tls = off.

We can test on nightly users, and if there are some providers which still do not support TLS 1.2, I think we can add an exception for them via provider database, applicable only to a single provider.

Even in 2021 TLS<1.2 websites are reported to be almost non-existent: https://www.f5.com/labs/articles/threat-intelligence/the-2021-tls-telemetry-report
No idea how applicable it is to email providers, but I expect that public providers keep more or less up to date.