chore: specify pnpm v8 as package manager (#2104)

This is especially useful for netlify which rely on corepack to use the proper pnpm version.
Corepack uses the `packageManager` field in the pakage.json file to determine which version is used.

We also need to use pnpm to build the docs on netlify, otherwise the deploy fails

netlify.toml

@@ -1,3 +1,6 @@
 [[redirects]]
   from="/v2"
   to="/"
+
+[build]
+  command = "pnpm docs:build"

package.json

@@ -114,6 +114,7 @@
     "url": "git+https://github.com/vuejs/test-utils.git"
   },
   "homepage": "https://github.com/vuejs/test-utils",
+  "packageManager": "pnpm@8.6.3",
   "pnpm": {
     "peerDependencyRules": {
       "ignoreMissing": [
@@ -124,4 +125,4 @@
       ]
     }
   }
-}
+}