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 <[email protected]>
Reviewed-By: Beth Griggs <[email protected]>
Reviewed-By: Richard Lau <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Gireesh Punathil <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>
Reviewed-By: Jiawen Geng <[email protected]>
Reviewed-By: Myles Borins <[email protected]>

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