ci: upgrade pnpm to version 9

- Add engines field to package.json
- Lock pnpm to version 9.0.6 with corepack

Author: thebuilder

package.json

@@ -27,7 +27,6 @@
     "url": "https://github.com/thebuilder/react-intersection-observer.git"
   },
   "engines": {
-    "node": ">=18",
     "pnpm": ">=9"
   },
   "packageManager": "pnpm@9.0.6+sha256.0624e30eff866cdeb363b15061bdb7fd9425b17bc1bb42c22f5f4efdea21f6b3",