GitHub actions: replace ubuntu-20.04 with ubuntu-latest (#4228)

GitHub will no longer support the Ubuntu 20.04 runner image starting
from April 1, 2025.

actions/runner-images#11101

Author: deniak

.github/workflows/11ty-publish.yaml

@@ -10,7 +10,7 @@ on:
 jobs:
   main:
     name: deploy (11ty)
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - name: Checkout the repository
         uses: actions/checkout@v4