The AEAD Encrypted Data packet has been removed from the draft
in favor of version 2 of the Sym. Encrypted Integrity Protected
Data packet. It also has a new feature flag to match.

This flag has been removed from the draft specification.

Also, set it as the preferred AEAD algorithm.

This subpacket replaces both symmetric algorithm preferences and
AEAD algorithm preferences when AEAD is supported, by providing
sets of preferred symmetric and AEAD algorithm pairs.

We still keep the symmetric algorithm preferences in case AEAD is
not supported.

Chrome's Web Crypto implementation doesn't support it, and it
seems unnecessary to list it when AES-256 is available.

This field was removed from the draft specification.

In unencrypted v5 secret key packets, don't include the count of
the optional fields, as per the updated crypto refresh.
Since they are always absent, the count is not needed.

This has been changed in the crypto refresh.

Key flags, expiration time, algorithm preferences, et cetera, are now
read from the direct-key signature instead of the primary User ID
binding signature for V5 keys.

This also requires a direct-key signature to be present for V5 keys.

The crypto refresh says that we MUST NOT reject messages where the
CRC24 checksum is incorrect. So, we remove the check for it.

Also, remove the checksumRequired config.

The latest crypto refresh specifies an HKDF step to be used for
deriving the key to encrypt the session key with.

It also specifies two additional length fields.