release: Remove references to ChallengeResponseAuthentication

This sshd_config keyword was replaced by KbdInteractiveAuthentication in
openssh 8.7, though ChallengeResponseAuthentication is silently accepted
as an alias. However, this means that the code in ec2.conf which
modifies a commented-out line no longer does anything. Apply a minimal
fix.

Reviewed by: cperciva, emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34400