ci: Use cilium v1.19 branch

Cilium/proxy v1.36 is used in cilium v1.19, but not main

Author: sayboras

.github/workflows/cilium-integration-tests.yaml

@@ -27,7 +27,7 @@ env:
   # renovate: datasource=github-releases depName=kubernetes-sigs/kind
   KIND_VERSION: v0.32.0
   CILIUM_REPO_OWNER: cilium
-  CILIUM_REPO_REF: main
+  CILIUM_REPO_REF: v1.19
   CILIUM_CLI_REF: latest
   CURL_PARALLEL: ${{ vars.CURL_PARALLEL || 10 }}