package/fetchmail: bump version to 6.4.21

Release notes:
https://sourceforge.net/p/fetchmail/mailman/message/37333073/

"It contains the security fix for CVE-2021-36386 of 6.4.20, and fixes
a regression/a bug that causes log message truncation/run-together
prominently visible with --logfile that was introduced into 6.4.20."

Updated note for CVE-2021-36386:
https://sourceforge.net/p/fetchmail/mailman/message/37333078/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>