Fix svc mode for chrome v113, change default svc mode to L3T3_KEY (#720)
* Fix svc mode for chrome v113.
Fix svc mode for chrome v113.
Add closable spatial layer for svc dynacast but disable it as two
blur/frozen issues
with current server/client:
1. chrome 113: when switching to up layer with scalability Mode change, it will generate a
low resolution frame and recover very quickly, but noticable
2. livekit sfu: additional pli request cause video frozen for a few frames, also noticable