feat(renovate): migrate to Renovate, retire Dependabot (#4754)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/dependabot.yml

@@ -1,14 +0,0 @@
-version: 2
-updates:
-  - package-ecosystem: "pip"
-    directory: "/"
-    schedule:
-      interval: "daily"
-    labels:
-      - "dependencies"
-  - package-ecosystem: "github-actions"
-    directory: "/"
-    schedule:
-      interval: "weekly"
-    labels:
-      - "dependencies"

renovate.json

@@ -0,0 +1,7 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "github>PrefectHQ/renovate-config",
+    "github>PrefectHQ/renovate-config:python"
+  ]
+}