Use Node20 for tests

A try following https://github.com/PrivateBin/PrivateBin/pull/1189#pullrequestreview-1695447526

.github/workflows/tests.yml

@@ -104,7 +104,7 @@ jobs:
     - name: Setup Node
       uses: actions/setup-node@v4
       with:
-        node-version: '16'
+        node-version: '20'
         cache: 'npm'
         cache-dependency-path: 'js/package-lock.json'