doc: mark querystring api as legacy

Signed-off-by: James M Snell <jasnell@gmail.com>
Fixes: #38231

PR-URL: #38436
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/querystring.md

@@ -2,7 +2,7 @@
 
 <!--introduced_in=v0.1.25-->
 
-> Stability: 2 - Stable
+> Stability: 3 - Legacy
 
 <!--name=querystring-->
 
@@ -15,6 +15,9 @@ query strings. It can be accessed using:
 const querystring = require('querystring');
 ```
 
+The `querystring` API is considered Legacy. While it is still maintained,
+new code should use the {URLSearchParams} API instead.
+
 ## `querystring.decode()`
 <!-- YAML
 added: v0.1.99