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: #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>

BUILDING.md

@@ -212,9 +212,10 @@ Supported platforms and toolchains change with each major version of Node.js.
 This document is only valid for the current major version of Node.js.
 Consult previous versions of this document for older versions of Node.js:
 
+* [Node.js 13](https://github.com/nodejs/node/blob/v13.x/BUILDING.md)
+* [Node.js 12](https://github.com/nodejs/node/blob/v12.x/BUILDING.md)
 * [Node.js 10](https://github.com/nodejs/node/blob/v10.x/BUILDING.md)
 * [Node.js 8](https://github.com/nodejs/node/blob/v8.x/BUILDING.md)
-* [Node.js 6](https://github.com/nodejs/node/blob/v6.x/BUILDING.md)
 
 ## Building Node.js on supported platforms