doc: remove faulty justification for 128-bit AES

This sentence implies that AES-128 is preferred over AES-256 because
of a related-key attack from 2009. However, that attack by Alex
Biryukov, Orr Dunkelman, Nathan Keller, Dmitry Khovratovich, and Adi
Shamir, while impressive, is only effective against variants of
AES-256 with a reduced number of rounds and it requires related keys.
This means that the attack is not effective against AES-256 as it is
used within TLS.

(AES-128 is still often preferred over AES-256 simply because it is
believed to be sufficiently secure and because it is faster.)

PR-URL: #42578
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Mestery <mestery@protonmail.com>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

Author: tniessen