Update Ubuntu version in GitHub Actions workflows

Update the `runs-on` value in the `.github/workflows/dobby-actions.yml` file to use `ubuntu-latest` instead of `ubuntu-20.04`.

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.github/workflows/dobby-actions.yml

@@ -1,4 +1,3 @@
-
 name: "Dobby action"
 on:
   issue_comment:
@@ -7,7 +6,7 @@ permissions:
   contents: read
 jobs:
   pr_commented:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     if: startsWith(github.event.comment.body, '/dobby')
     env:
       BUNDLE_WITHOUT: "development:test"