Support tls 1.3 in TLS configuration options

[dopey]

Now being on Go 1.13+ we can provide TLS 1.3 support. Let's make this available.

[maraino]

Now that we're already enforcing Go 1.13+ we should be able to do this.

[dopey]

@maraino anything holding us back from making this the new default?

[maraino]

@dopey I've been testing for a long "minVersion": 1.2, "maxVersion": 1.3 without any issues