Update runner for backport workflow (#8194)

Author: jander-msft

.github/workflows/backport.yml

@@ -8,7 +8,7 @@ permissions: {}
 jobs:
   backport:
     if: github.event.issue.pull_request != '' && contains(github.event.comment.body, '/backport to') && contains(fromJSON('["OWNER", "MEMBER", "COLLABORATOR"]'), github.event.comment.author_association)
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     permissions:
       contents: write
       issues: write