BLAKE3 is generally faster and provides a more modern construction for
keyed hashing (using its internal keyed mode instead of the construction
used for BLAKE2b).
Key types changed:
- authenticated-blake2 -> authenticated-blake3
- {keyfile,repokey}-blake2-aes-ocb -> {keyfile,repokey}-blake3-aes-ocb
- {keyfile,repokey}-blake2-chacha20-poly1305 -> {keyfile,repokey}-blake3-chacha20-poly1305
This also fixes the slightly unusual way how we used blake2b,
it is only supported for importing borg 1.x repos.
New repos either use HMAC-SHA256 or BLAKE3.
