Update markdownlint.yml (#40336)

Related to dotnet/docs-aspire#567 and discovered in https://github.com/dotnet/docs/actions/runs/8565576901/job/23473831814?pr=40328

.github/workflows/markdownlint.yml

@@ -25,9 +25,9 @@ jobs:
     steps:
       - uses: actions/[email protected]
       - name: Use Node.js
-        uses: actions/setup-node@60edb5dd545a775178f52524783378180af0d1f8
+        uses: actions/setup-node@c2ac33f2c62f978d6c944d9648125a294e56dc0b
         with:
-          node-version: 16.x
+          node-version: 20.x
       - name: Run Markdownlint
         run: |
           echo "::add-matcher::.github/workflows/markdownlint-problem-matcher.json"