doc: remove Node.js 20 link from BUILDING

Signed-off-by: Mike McCready <66998419+MikeMcC399@users.noreply.github.com>

Author: MikeMcC399

BUILDING.md

@@ -232,7 +232,6 @@ Consult previous versions of this document for older versions of Node.js:
 
 * [Node.js 24](https://github.com/nodejs/node/blob/v24.x/BUILDING.md)
 * [Node.js 22](https://github.com/nodejs/node/blob/v22.x/BUILDING.md)
-* [Node.js 20](https://github.com/nodejs/node/blob/v20.x/BUILDING.md)
 
 ## Building Node.js on supported platforms