doc: add 13 and 12 to previous versions

Remove Node.js 6 and add Node.js 12 and Node.js 13 to the previous
versions of this document section

PR-URL: https://github.com/nodejs/node/pull/30590


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>