Migrate to Active Record Encryption

[aergonaut]

attr_encrypted raises some strange errors when trying to upgrade to Rails 7.

Given that Active Record Encryption exists, it would be good to migrate off of attr_encrypted and use Active Record Encryption instead.

One wrinkle will be migrating existing access keys. We could wipe out the column entirely and then invalidate all session tokens to force users to re-authenticate and refresh their access keys.