ci: switch from Dependabot to Renovate

Replace Dependabot with Renovate for more comprehensive dependency management:
- Auto-update Go version in go.mod via golang-version datasource
- Auto-update Dockerfile base images via customManagers
- Auto-update GitHub Actions versions
- Group related dependencies (kubernetes, golang, openshift, prometheus)
- Run go mod tidy after updates automatically

Also bump Go version from 1.25.0 to 1.25.5.

Author: jimyag

.github/dependabot.yml

@@ -1,6 +1,6 @@
 module github.com/jimyag/auto-cert-webhook
 
-go 1.25.0
+go 1.25.5
 
 require (
 	github.com/appscode/jsonpatch v1.0.1

go.mod

@@ -0,0 +1,55 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "config:recommended",
+    "customManagers:dockerfileVersions",
+    "customManagers:githubActionsVersions"
+  ],
+  "postUpdateOptions": [
+    "gomodTidy"
+  ],
+  "labels": [
+    "dependencies"
+  ],
+  "packageRules": [
+    {
+      "matchPackageNames": [
+        "k8s.io/**",
+        "sigs.k8s.io/**"
+      ],
+      "groupName": "kubernetes"
+    },
+    {
+      "matchPackageNames": [
+        "golang.org/x/**",
+        "go"
+      ],
+      "groupName": "golang"
+    },
+    {
+      "matchPackageNames": [
+        "github.com/openshift/**"
+      ],
+      "groupName": "openshift"
+    },
+    {
+      "matchPackageNames": [
+        "github.com/prometheus/**"
+      ],
+      "groupName": "prometheus"
+    },
+    {
+      "matchDatasources": [
+        "golang-version"
+      ],
+      "rangeStrategy": "bump",
+      "enabled": true
+    },
+    {
+      "matchUpdateTypes": [
+        "digest"
+      ],
+      "enabled": false
+    }
+  ]
+}