Remove non-standard ACME fields from WFE2 responses

We'd like to start deprecating and removing non-standard fields from Boulder's responses.

Checklist of wire-level fields Boulder emits that are not defined in the RFCs or the drafts Boulder implements: RFC 8555, RFC 9773, RFC 8737, draft-ietf-acme-profiles, draft-ietf-acme-dns-account-challenge, draft-ietf-acme-dns-persist).

• Account key — emits the full JWK on every account response. RFC 8555
  §7.1.2 enumerates exactly status, contact, termsOfServiceAgreed,
  externalAccountBinding, orders; key is not defined, and §7.3
  specifies the server "MUST NOT reflect … any unrecognized fields" in the
  resulting account object.
  core.Registration.Key

• Account createdAt — non-standard timestamp on account responses.
  Not defined in RFC 8555 §7.1.2.
  core.Registration.CreatedAt

• Challenge validationRecord — serializes an internal object tree
  (URLs, hostnames, ports, resolved/tried IPs, resolver addresses) onto
  challenge responses. RFC 8555 §7.1.5 / §8 define only type, url,
  status, validated, error plus per-type fields; this field is not
  defined for any challenge type in RFC 8555, RFC 8737, or the dns-account
  / dns-persist drafts.
  core.Challenge.ValidationRecord

Start in core/objects.go at Registration.Key, Registration.CreatedAt, and Challenge.ValidationRecord, then trace how these fields are serialized into WFE2 responses. Use the cited RFC sections to confirm the allowed response fields. Done means the three non-standard fields no longer appear in the relevant account and challenge responses, with coverage for the changed wire format.