doc: update BUILDING.md previous versions links

PR-URL: #52852
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

Author: targos

BUILDING.md

@@ -222,9 +222,9 @@ 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 19](https://github.com/nodejs/node/blob/v19.x/BUILDING.md)
+* [Node.js 21](https://github.com/nodejs/node/blob/v21.x/BUILDING.md)
+* [Node.js 20](https://github.com/nodejs/node/blob/v20.x/BUILDING.md)
 * [Node.js 18](https://github.com/nodejs/node/blob/v18.x/BUILDING.md)
-* [Node.js 16](https://github.com/nodejs/node/blob/v16.x/BUILDING.md)
 
 ## Building Node.js on supported platforms