chore: fix pnpm requirements in CONTRIBUTING.md

use pnpm other than 9.x will report an error.

ref commit:
silverhand-io/essentials@2171b96

Author: xiaohanyu

.github/CONTRIBUTING.md

@@ -65,7 +65,7 @@ We use the monorepo approach for development. Since pnpm supports monorepo natur
 You'll need these installed to proceed:
 
 - [Node.js](https://nodejs.org/) `^18.12.0`
-- [pnpm](https://pnpm.io/) `^7.0`
+- [pnpm](https://pnpm.io/) `^9.0`
 - A [Postgres](https://postgresql.org/) `^14.0` instance
 
 ### Clone and install dependencies