Highest keyset id version byte this build can spend: 0x01, the v2 keysets.
Remarks
deriveKeysetId below is the authority on which versions exist, so bump this only alongside a
new case there. Named for the byte, not the keyset generation: the docs call byte 0x01 a "v2"
keyset, so an unqualified number here would be read one generation out.
Highest keyset id version byte this build can spend: 0x01, the v2 keysets.