tools: use Node.js 16.x for GitHub workflow

find-inactive-collaborators.mjs works fine with Node.js 16.x, but GitHub
Actions currently use 14.x by default.

PR-URL: https://github.com/nodejs/node/pull/39362


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>