ci(preview): add beta branch and update runner image

Author: lorenzocorallo

.github/workflows/preview.yml

@@ -3,6 +3,7 @@ on:
   push:
     branches:
       - 'main'
+      - 'beta'
   pull_request_target:
     types:
       - opened
@@ -17,7 +18,7 @@ concurrency:
 jobs:
   preview:
     name: Preview
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     env:
       PREVIEW_REPO: PoliNetworkOrg/preview
       PAGES_BASE: https://PoliNetworkOrg.github.io/preview