doc: v6 is now LTS rather than Current

PR-URL: https://github.com/nodejs/node/pull/9182


Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>