Fixes #35794 - change bootdisk_allowed_types type to array

The setting value shows up as "subnetfull_host" whereas if we edit the value,
we will be able to see that it is set as "subnet,full_host"

(cherry picked from commit 7c75d4d)

lib/foreman_bootdisk/engine.rb

@@ -124,7 +124,7 @@ class Engine < ::Rails::Engine
               description: N_("Installation media files will be cached for full host images")
 
             setting "bootdisk_allowed_types",
-              type: :string,
+              type: :array,
               default: Setting::Bootdisk.bootdisk_types,
               full_name: N_("Allowed bootdisk types"),
               description: N_("List of allowed bootdisk types, remove type to disable it")