Configure renovate

Since we are using the renovate app now, remove the global `platform` and `onboarding` settings.

Use our preset rules to make sure only dependencies of test modules are updated automatically, but
the root module uses the minimally required version.

Author: avdv

.github/renovate.json

@@ -1,13 +1,8 @@
 {
-    "enabledManagers": [
-        "bazel",
-        "bazel-module"
-    ],
     "extends": [
-        "config:recommended"
+        "github>tweag/renovate-presets:ruleset_base",
+        "github>tweag/renovate-presets:mergify"
     ],
     "ignorePaths": ["registry/**"],
-    "gitAuthor": "Renovate Bot <bot@renovateapp.com>",
-    "onboarding": false,
-    "platform": "github"
+    "gitAuthor": "Renovate Bot <bot@renovateapp.com>"
 }