multimedia/svt-av1-psy: switch to an active fork

https://github.com/psy-ex/svt-av1-psy/commit/21db3f763811
"This repository was archived by the owner on Apr 30, 2025. It is now read-only."

Reported by:	mr44er (via private mail)
Tested by:	diff -urN (top directory rename only)