Switch from ubuntu-20.04 -> ubuntu-lastest

Ubuntu-20.04 was retired in March 2025 and therefore GitHub Actions
are not executed at all

Signed-off-by: Petr "Stone" Hracek <phracek@redhat.com>

Author: phracek

.github/workflows/openshift-tests.yml

@@ -6,7 +6,7 @@ jobs:
   openshift-tests:
     # This job only runs for '[test] pull request comments by owner, member
     name: "RHEL9 tests: imagestream ${{ matrix.version }}"
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     strategy:
       fail-fast: false
       matrix: