chore(renovate): hold pnpm below 12

The pinned pnpm 11 in workflow-prepare.sh and ci.yaml cannot switch to the
pnpm 12 native binary. Node runs the binary as JavaScript and the install fails.

renovate.json

@@ -0,0 +1,11 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "packageRules": [
+    {
+      "description": "pnpm 11 cannot switch to the pnpm 12 native binary: npx runs it with node",
+      "matchDepTypes": ["packageManager"],
+      "matchPackageNames": ["pnpm"],
+      "allowedVersions": "<12"
+    }
+  ]
+}