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)