Upgrade postcss-nesting to version 14.0.0

This is a major version bump, but the changelog at
https://github.com/csstools/postcss-plugins/blob/main/plugins/postcss-nesting/CHANGELOG.md
doesn't indicate any breaking changes that should impact us.

We only have to list `postcss-nesting` in `eslint.config.js` similar
to e.g. commit a2909f9.

Author: timvandermeij

eslint.config.mjs

@@ -118,6 +118,7 @@ export default [
             "web",
             "fluent-bundle",
             "fluent-dom",
+            "postcss-nesting",
             "postcss-values-parser",
             "stylelint",
             // See https://github.com/firebase/firebase-admin-node/discussions/1359.

package-lock.json

@@ -46,7 +46,7 @@
     "postcss": "^8.5.6",
     "postcss-dir-pseudo-class": "^9.0.1",
     "postcss-discard-comments": "^7.0.5",
-    "postcss-nesting": "^13.0.2",
+    "postcss-nesting": "^14.0.0",
     "postcss-values-parser": "^7.0.0",
     "prettier": "^3.8.1",
     "puppeteer": "^24.37.3",