deploy-docs.yml: Update Node.js 12 actions to Node.js 16 actions

This change is made as Node.js 12 is now depricated

.github/workflows/deploy-docs.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout main
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Deploy docs
         uses: mhausenblas/mkdocs-deploy-gh-pages@master