doc: mark querystring api as legacy

Signed-off-by: James M Snell <jasnell@gmail.com>
Fixes: https://github.com/nodejs/node/issues/38231

PR-URL: https://github.com/nodejs/node/pull/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>