chore: run tests with NodeJS 14

I expect no stuff to break or so, so let's just try to use the current recommend LTS version. (v14 will also die at some time, but Fedora e.g. still seems to use it for now by default. Likely we may upgrade soon even more.)

Ref https://nodejs.org/en/about/releases/

.github/workflows/tests.yml

@@ -104,7 +104,7 @@ jobs:
     - name: Setup Node
       uses: actions/setup-node@v3
       with:
-        node-version: '12'
+        node-version: '14'
         cache: 'npm'
         cache-dependency-path: 'js/package.json'