[3.13] GH-105879: Note exec/eval keyword change in What's New (GH-121831) (#121852)

GH-105879: Note exec/eval keyword change in What's New (GH-121831)
(cherry picked from commit d7a099d7ae749798bd822aee5d1d46d0a335189b)

Co-authored-by: Alyssa Coghlan <ncoghlan@gmail.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>