net/asterisk20: Update 20.20.1 => 20.21.0

Changelog:
https://downloads.asterisk.org/pub/telephony/asterisk/releases/ChangeLog-20.21.0.html

PR: 298203
Reported by: Oleksandr Kryvulia <o.kryvulia@flex-it.com.ua> (maintainer)
Approved by: osa, vvd (Mentors, implicit)
Sponsored by: FLEX-IT LLC
MFH: 2026Q3

(cherry picked from commit a1c4b69761ea3d6049db512a9430a0dfd07c885f)