Certificate creation API returns 400 if `letsencrypt_agree` or `letsencrypt_email` are included — undocumented breaking change

[katelovescode]

As of v2.14.0, passing letsencrypt_agree or letsencrypt_email in the
POST /api/nginx/certificates request body causes a 400 with no descriptive
error message. These fields were accepted in prior versions and are still
referenced in community documentation and tooling.

Observed Impact: Cert creation failed with a generic 400 after upgrading,
with no indication which field is the problem.

Request:

1. Add a note to the v2.14.0 release notes that these fields were removed.
2. Return a descriptive error message naming the offending field rather than a
   generic 400.

Tested against v2.14.0.