I'm creating this issue here instead of the website repo since we use this issue tracker more.

CentOS 6 reaches its EOL at the end of November so we should remove installation instructions for CentOS/RHEL 6 from the website.

It's important to do this before our December release because we're planning to fully deprecate certbot-auto then and the deprecation message tells users to visit the Certbot website where we currently tell these users to use certbot-auto.