ci: add K8s v1.31 to integration test matrix

The live OKE cluster runs v1.31.10 but integration tests only covered
v1.28-v1.30. Adds kindest/node:v1.31.14 to close the coverage gap.

Author: privsim

.github/workflows/integration-test.yml

@@ -16,6 +16,7 @@ jobs:
           - v1.28.13
           - v1.29.8
           - v1.30.4
+          - v1.31.14
     steps:
       - uses: actions/checkout@v4