[9.16] Deprecate AES algorithm for DNS cookies

The AES algorithm for DNS cookies was being kept for legacy reasons, and it can be safely removed in the next major release. Mark is as deprecated, so the named-checkconf prints a warning when in use.

(cherry picked from commit 67d14b0e)

Closes #4421 (closed)

Backport of !8472 (merged)

Edited by Ondřej Surý

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