update node version for eslint

Github is updating all their actions to node 20, may as well do the same
here? Not strong need to update it, just spotted this.

.github/workflows/ci.yml

@@ -50,7 +50,7 @@ jobs:
           python-version: '3.10'
       - uses: actions/setup-node@v4
         with:
-          node-version: '16.x'
+          node-version: '20.x'
         if: "matrix.testenv == 'eslint'"
       - name: Set up problem matchers
         run: "python scripts/dev/ci/problemmatchers.py ${{ matrix.testenv }} ${{ runner.temp }}"