net/asterisk20: Update 20.15.1 => 20.15.2 (security fix)

Changelog:
https://downloads.asterisk.org/pub/telephony/asterisk/releases/ChangeLog-20.15.2.html

GHSA-64qc-9x89-rx5j: A specifically malformed Authorization header in
an incoming SIP request can cause Asterisk to crash.

PR: 289180
Security: GHSA-64qc-9x89-rx5j
Sponsored by: FLEX-IT LLC
MFH: 2025Q3

(cherry picked from commit ffcdceaf3b4e9ceaa762473454292d2c7ca758b0)