CI: Update deploy.yml

Increase actions/checkout dependency version.

.github/workflows/deploy.yml

@@ -2,15 +2,15 @@ name: Deploy to WordPress.org
 on:
   release:
     types: [released]
-  # Allow manually triggering the workflow.
+  # Allow manual triggering of the workflow.
   workflow_dispatch:
 jobs:
   release:
     name: New release to WordPress.org
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Push to WordPress.org
         uses: 10up/action-wordpress-plugin-deploy@stable
         env: