build: Move to a webpack module

Cockpit recently changed to an ESM build system [1]. Bump
COCKPIT_REPO_COMMIT to that and follow suit.

This does not work with old node.js 10 any more which is still the
default in RHEL 8. Install the newer version 16 instead.

[1] https://github.com/cockpit-project/cockpit/pull/18366

package.json

@@ -1,6 +1,7 @@
 {
   "name": "starter-kit",
   "description": "Scaffolding for a cockpit module",
+  "type": "module",
   "main": "index.js",
   "repository": "git@github.com:cockpit/starter-kit.git",
   "author": "",