bump version for 0.21 (risc0#1544)

bump version to 0.21.0-rc.2 (risc0#1535)

bump version to 0.21.0-rc.1 (risc0#1496)

Refactor to allow bonsai-sdk to be published on crates.io

Release v0.20.0 (risc0#1386)

Bump version to v0.20.0-rc.4

Bump version to v0.20.0-rc.3 (risc0#1375)

Update ImageIDs

Fix release token (risc0#1252) (risc0#1253)

Release v0.19.1 (risc0#1134)