Add notice for end of support

[thibmeu]

Privacy Pass has changed since the original PoPETS version. Moreover, CAPTCHA providers have moved from that version to follow IETF version more closely. They don't operate with a stored token model, but with an interactive token issuance and redemption, with a split attester/issuer model. This commit updates the README to highlight the end of active token issuer/redeemer for this specific version of the protocol.