ci: add k8s v1.31 and remove v1.28 from stackit e2e test (#3699)

Signed-off-by: Daniel Weiße <dw@edgeless.systems>

Author: daniel-weisse

.github/workflows/e2e-test-stackit.yml

@@ -40,7 +40,7 @@ jobs:
       fail-fast: false
       max-parallel: 6
       matrix:
-        kubernetesVersion: [ "1.28", "1.29", "1.30" ]
+        kubernetesVersion: [ "1.29", "1.30", "1.31" ]
         clusterCreation: [ "cli", "terraform" ]
         test: [ "sonobuoy quick" ]
     runs-on: ubuntu-24.04