Update Node.js version to 21.x (#45)

Author: garrytrinder

.github/workflows/pr.yml

@@ -17,7 +17,7 @@ jobs:
       - name: Install Node.js
         uses: actions/setup-node@v4
         with:
-          node-version: 18.x
+          node-version: 21.x
       - run: npm install
       - run: xvfb-run -a npm test
         if: runner.os == 'Linux'