chore: pin pnpm version (#1950)

">=9" is was making renovate bot runner choose v10 that has some breaking changes and messes up lock file when updating dependencies

Updating to v10 is not possible because Cloudflare still uses v9 and can't handle v10 lock file and fails when deploying website app.

Author: YaroShkvorets

package.json

@@ -13,7 +13,7 @@
   "private": true,
   "engines": {
     "node": ">=20.0.0",
-    "pnpm": ">=9"
+    "pnpm": "9"
   },
   "scripts": {
     "build": "pnpm --filter=@graphprotocol/graph-* build",