CI: stop testing on Node 16

[Node 16 has been unsupported since September.][0] We will drop support
in Helmet 8. For now, we can stop testing it in CI.

[0]: https://nodejs.org/en/blog/announcements/nodejs16-eol/

.github/workflows/nodejs.yml

@@ -11,7 +11,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [16.x, 18.x, 20.x]
+        node-version: [18.x, 20.x]
 
     steps:
       - uses: actions/checkout@v4