Relax version requirements for npm

The strict version causes issues on GitHub CI when running on Windows,
as we require a strict 11.6.2 version but the installed version is 11.8.0:
- https://github.com/alphagov/govuk-frontend/actions/runs/22199182869/job/64207330184?pr=6756
- https://github.com/alphagov/govuk-frontend/actions/runs/22193376028/job/64187136429?pr=6755

This is because the container runs Node 24.13.1, which now comes with npm 11.8.0:
nodejs/node#61466 (via https://nodejs.org/en/blog/release/v24.13.1).

Author: romaricpascal

docs/examples/webpack/package.json

@@ -4,7 +4,7 @@
   "description": "Boilerplate example for govuk-frontend using webpack",
   "engines": {
     "node": "^24.11.0",
-    "npm": "11.6.2"
+    "npm": "^11.6.2"
   },
   "scripts": {
     "dev": "webpack serve --progress --mode development",