Check for SEIPDv2 Feature Flag in `rnp_ctx_t::pkeskv6_capable()`

[TJ-91]

Currently, it only takes into account the recipients' key versions.

[TJ-91]

Crypto Refresh:

When encrypting to one or more public keys:

If all recipient keys indicate support for version 2 of the Symmetrically Encrypted Integrity Protected Data packet in their Features subpacket (Section 5.2.3.32), or are v6 keys without a Features subpacket, or the implementation can otherwise infer that all recipients support v2 SEIPD packets, the implementation SHOULD encrypt using a v2 SEIPD packet.

If one of the recipients does not support v2 SEIPD packets, then the message generator MAY use a v1 SEIPD packet instead.