[3.13] gh-88726: Stop using non-standard charset names eucgb2312_cn and big5_tw in email (GH-149959) (GH-150493)

(cherry picked from commit 5e467f4331d4cb7a8e2986c27af7eb68ccaccb37)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>