net/freerdp3: Update 3.20.2 => 3.21.0 (8 CVEs)

Changelog:
https://github.com/FreeRDP/FreeRDP/releases/tag/3.21.0

Security: CVE-2026-23530
Security: CVE-2026-23531
Security: CVE-2026-23532
Security: CVE-2026-23533
Security: CVE-2026-23534
Security: CVE-2026-23732
Security: CVE-2026-23883
Security: CVE-2026-23884
MFH: 2026Q1