fix(ci): let pnpm version be read from package.json packageManager field

The pnpm/action-setup detects version from package.json's packageManager
field automatically. Specifying version in workflow causes conflict.

.forgejo/workflows/deploy-frontend.yml

@@ -28,7 +28,6 @@ on:
 
 env:
   NODE_VERSION: '20'
-  PNPM_VERSION: '9'
   SERVER_IP: '91.98.224.44'
   SERVER_USER: 'root'
 
@@ -50,8 +49,7 @@ jobs:
 
       - name: Install pnpm
         uses: https://github.com/pnpm/action-setup@v4
-        with:
-          version: ${{ env.PNPM_VERSION }}
+        # Version is read from package.json packageManager field
 
       - name: Setup Node.js
         uses: actions/setup-node@v4