renovate.json: adjust managers

We can apparently use the pep621 manager to manage uv dependencies
now. And we need to disable the pip_requirements manager so the bot
doesn't try to update our requirements.txt file (which is exported
from uv).

renovate.json

@@ -3,7 +3,7 @@
   "extends": [
     "config:recommended"
   ],
-  "pep621": {
+  "pip_requirements": {
-    "enabled": false
+      "enabled": false
   }
 }