Update CONTRIBUTING with node 16 requirement

Since node 12 was deprecated by GitHub

Author: rffontenelle

CONTRIBUTING.md

@@ -2,7 +2,7 @@
 If you plan to contribute to flatpak-github-actions, here's a couple of things that could help you get started.
 
 ## Prerequisites
-- NodeJS 12.x or newer
+- NodeJS 16.x or newer
 - Yarn
 - `vercel/ncc` you can install it with `yarn global add vercel/ncc`