Update to 9.18.21
    
    https://downloads.isc.org/isc/bind9/9.18.21/doc/arm/html/notes.html#notes-for-bind-9-18-21
    
    Removed Features
    
    - Support for using AES as the DNS COOKIE algorithm (cookie-algorithm aes;) has been deprecated and will be removed in a future release. Please use the current default, SipHash-2-4, instead. [GL #4421]
    
    - The resolver-nonbackoff-tries and resolver-retry-interval statements have been deprecated. Using them now causes a warning to be logged. [GL #4405]