Replace AV1 with AV2

[flibitijibibo]

The AV2 spec was released back in May...

https://av2.aomedia.org/

... and since we haven't committed to AV1 yet, maybe we can jump straight to 2 and reduce the support surface a bit.

CC @thatcosmonaut, @kiddkaffeine

[thatcosmonaut]

The dav2d reference decoder is still extremely early, I wouldn't expect it to be ready for prime time for a while.