Fix symmetric v6 message encryption

[vanitasvitae]

Commits 7d95b08 and ea31631 introduced an error where the plain session key was passed in the wrong format causing the session-key wrapper to fail due to an invalid block size.

[ligefeiBouncycastle]

• As stated in RFC 9580, Section 3.7.2.1, Table 2, the Encryption column for 253 (AEAD) specifies the format as AEAD(HKDF(S2K(passphrase), info), secrets, packetprefix). The session key without checksum as part of its structure.
• The sessionInfo is a constructed representation where a 1-byte algorithm identifier is prepended to the sessionKey, followed by a 2-byte checksum. In the original implementation of PBEKeyEncryptionMethodGenerator.generateV6ESK, there was a step to strip these additional 3 bytes from sessionInfo before usage.
  To optimize the process and align with the documentation, the decision was made to use the sessionKey directly for version 6 packages (v6 package), bypassing the intermediate sessionInfo structure.

[vanitasvitae]

Hm, apparently I forgot to implement PKESKv6 generation completely :D
I'll work on a fix.