Upgrade tested node version to 20.x

tc-share has been [upgraded](conversation/tc-share#219) to 20.x, so we
should test this library against 20.x. Not doing so is
[preventing](https://github.com/conversation/ShareJS/actions/runs/9809447104/job/27087401539?pr=48)
[upgrades](https://github.com/conversation/ShareJS/actions/runs/9809447160/job/27362292538?pr=49)
which rely on using node 18.x or above.

.github/workflows/test.yml

@@ -20,7 +20,7 @@ jobs:
       - name: Use Node.js
         uses: actions/setup-node@v2
         with:
-          node-version: '16.x'
+          node-version: '20.x'
       - run: |
           sudo apt-get install postgresql libpq-dev
           sudo service postgresql start