Changelog note for #762.

Changelog note for #762.
- Merge PR #762: Downstream DNS Server Cookies a la RFC7873 and
  RFC9018. Create server cookies for clients that send client cookies.
  This needs to be explicitly turned on in the config file with:
  `answer-cookie: yes`. A `cookie-secret:` can be configured for
  anycast setups. Without one, a random cookie secret is generated.
  The acl option `allow_cookie` allows queries with either a valid
  cookie or over a stateful transport. The statistics output has
  `queries_cookie_valid` and `queries_cookie_client` and
  `queries_cookie_invalid` information. The `ip\-ratelimit\-cookie:`
  value determines a rate limit for queries with cookies, if desired.