dev: use dependabot instead of renovate

Renovate lacks of commits diff: it's difficult
to ensure that update is safe.

.dependabot/config.yml

@@ -0,0 +1,10 @@
+version: 1
+update_configs:
+  - package_manager: "go:modules"
+    directory: "/"
+    update_schedule: "weekly"
+    default_labels:
+      - "dependency"
+    ignored_updates:
+      - match: # ignore forked linters because of their versioning issues. TODO: fix it.
+          dependency_name: "github.com/golangci/*"

.github/renovate.json

@@ -1,4 +0,0 @@
-{
-  "ignorePaths": ["docs/**"],
-  "labels": ["dependency"]
-}