docs: correct pnpm version (#8807)

.github/contributing.md

@@ -57,7 +57,7 @@ Hi! I'm really excited that you are interested in contributing to Vue.js. Before
 
 ## Development Setup
 
-You will need [Node.js](https://nodejs.org) **version 16+**, and [PNPM](https://pnpm.io) **version 7+**.
+You will need [Node.js](https://nodejs.org) **version 16+**, and [PNPM](https://pnpm.io) **version 8+**.
 
 We also recommend installing [ni](https://github.com/antfu/ni) to help switching between repos using different package managers. `ni` also provides the handy `nr` command which running npm scripts easier.