ci: backport_issue_check: Use ubuntu-22.04 virtual environment

This commit updates the pull request backport issue check workflow to
use the Ubuntu 22.04 virtual environment.

Signed-off-by: Stephanos Ioannidis <[email protected]>
(cherry picked from commit cadd6e6)

Author: stephanosio

.github/workflows/backport_issue_check.yml

@@ -8,7 +8,7 @@ on:
 jobs:
   backport:
     name: Backport Issue Check
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
 
     steps:
       - name: Check out source code