use correct kex for RedHat/CentOS/Oracle Linux 8

Signed-off-by: Martin Schurz <Martin.Schurz@t-systems.com>

Author: schurzi

libraries/ssh_crypto.rb

@@ -96,8 +96,10 @@ def valid_kexs # rubocop:disable Metrics/CyclomaticComplexity, Metrics/AbcSize,
       case inspec.os[:release]
       when /^6\./
         kex = nil
-      when /^7\./, /^8\./
+      when /^7\./
         kex = kex66
+      when /^8\./
+        kex = kex80
       end
     # https://pkgs.alpinelinux.org/packages?name=openssh
     when 'arch'