feat(renovate): track oldest maintained k8s (#13670)

Signed-off-by: kiblik <5609770+kiblik@users.noreply.github.com>

Author: kiblik

.github/renovate.json

@@ -28,6 +28,15 @@
     "matchPackageNames": "renovatebot/renovate",
     "schedule": ["* * * * 0"]
   }],
+  "customDatasources": {
+    "endoflife-oldest-maintained": {
+      "defaultRegistryUrlTemplate": "https://endoflife.date/api/v1/products/{{packageName}}",
+      "format": "json",
+      "transformTemplates": [
+        "{ \"releases\": [$.result.releases[isMaintained = true]^(<eolFrom)[0].latest.{\"version\": name, \"releaseTimestamp\": date, \"changelogUrl\": link}], \"sourceUrl\": \"https://github.com/kubernetes/kubernetes\", \"homepage\": \"https://kubernetes.io/\" }"
+      ]
+    }
+  },
   "customManagers": [
     {
       "customType": "regex",

.github/workflows/k8s-tests.yml

@@ -18,7 +18,7 @@ jobs:
           # are tested (https://kubernetes.io/releases/)
           - k8s: 'v1.34.0' # renovate: datasource=github-releases depName=kubernetes/kubernetes versioning=loose
             os: debian
-          - k8s: 'v1.31.13' # Do not track with renovate as we likely want to rev this manually
+          - k8s: 'v1.31.13' # renovate: datasource=custom.endoflife-oldest-maintained depName=kubernetes
             os: debian
     steps:
       - name: Checkout