Make minimum DTLS version configurable

[algesten]

By default we should disallow DTLS 1.0. To do this we need a PR to land in rust-openssl crate: sfackler/rust-openssl#1886

The man page in openssl: https://www.openssl.org/docs/man3.1/man3/DTLSv1_2_method.html - tells us DTLSv1_2_method is deprecated. The way to limit the DTLS version (or TLS for that matter), is to use SSL_CTX_set_min_proto_version. In the Rust wrapper of openssl this corresponds to https://docs.rs/openssl/0.10.50/openssl/ssl/struct.SslContextBuilder.html#method.set_min_proto_version however SslVersion constant lacks the values we need: https://docs.rs/openssl/0.10.50/openssl/ssl/struct.SslVersion.html

[xnorpx]

Should be released now?

https://docs.rs/openssl/0.10.54/openssl/ssl/struct.SslVersion.html

[xnorpx]

@algesten could this be closed now?

[algesten]

It's not configurable yet. We do lock it down however.

[algesten]

Let's continue this in #326