http2: refactor to use primordials instead of <string>.indexOf

PR-URL: https://github.com/nodejs/node/pull/36679


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Pooja D P <Pooja.D.P@ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>