What's the best way to set up a web server to only use TLS 1.2?

In both Nginx and Apache you can specify which SSL or TLS protocols to use in the virtual host configuration for a specific website, or in the overall configuration for HTTPS connections.

For Apache you can use the following directive:

SSLProtocol all -SSLv2 -SSLv2 -TLSv1.0 -TLSv1.1

You can also reverse it, as so:

SSLProtocol -all TLSv1.2

There's more information on configuring SSL certificates and mod_ssl in Apache here.

For Nginx you use a slightly different directive:

ssl_protocols TLSv1.2;

There's an official guide on configuring HTTPS in Nginx here.