Follow-up from "feat: Lara 9 / php 8"

The following discussion from !12 (merged) should be addressed:

• @sam started a discussion:

  Since this is a major version, we should consider taking the opportunity to correct the naming of this field from rsa_key_id to keypair_id (and using keypair, or asymmetric key where appropriate). Reason being is that RSA is only one of the asymmetric key algorithms that can be used with this library.

  In fact we should consider using ed25519 instead of rsa-2048 as it's as secure, more performant, and has smaller keys.