Hardcode the value of SslProtocols.Tls13 for frameworks that do not support it.
Hardcode the value of SslProtocols.Tls13 for frameworks that do not support it.

This makes it possible for applications making use of the netstandard2.0 versions
of MailKit to get TLS v1.3 support if their underlying platforms support it.

Thanks to Theodore Tsirpanis for this patch.

Completes the fix for issue #1058

jstedfast/MailKit