chore: harden npm supply chain controls (#11157)

docs/user-guide/providers/llm/getting-started-llm.mdx

@@ -22,7 +22,7 @@ Install promptfoo using one of the following methods:
 
 **Using npm:**
 ```bash
-npm install -g promptfoo
+npm install --global promptfoo@0.121.11
 ```
 
 **Using Homebrew (macOS):**