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 <root@stephanos.io>
(cherry picked from commit cadd6e6fa4d801ee09f73332e8fe342df5493e7e)

.github/workflows/backport_issue_check.yml

diff --git a/.github/workflows/backport_issue_check.yml b/.github/workflows/backport_issue_check.yml
index 89b1d07..92e3375 100644
--- a/.github/workflows/backport_issue_check.yml
+++ b/.github/workflows/backport_issue_check.yml
@@ -8,7 +8,7 @@
 jobs:
   backport:
     name: Backport Issue Check
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
 
     steps:
       - name: Check out source code