Fix compatibility claims: require Go 1.14+
Fix compatibility claims: require Go 1.14+

Using `tls.CipherSuiteName()` meant that 1.14 became the minimum supported
version of Go.  Since that's the minimum supported upstream, I can live with
that, but I'm annoyed at myself for not catching that this was a min-req bump.

PennockTech/smtpdane,PennockTech/smtpdane