Revoke an issued credential identified by recipient email + credential
identifier. The credential expires immediately (its expiration date
becomes today in UTC) and its public verification page shows it as expired;
nothing is deleted. If the recipient holds more than one credential of
this template, every active one is revoked. Revoking a credential that is
already expired changes nothing and returns revoked: 0. Revoking does
not consume credits.

When the recipient has several credentials of this template, the response
describes the most recently issued one this request revoked or, when none
was still active, the most recently issued one.