Move to inline settings validations

This makes it clearer how the setting is exactly defined. As a bonus,
this syntax is compatible with both Ruby 2 and 3.

lib/foreman_discovery/engine.rb

@@ -75,11 +75,10 @@ class Engine < ::Rails::Engine
             setting "discovery_hostname",
               type: :array,
               default: ["discovery_bootif"],
+              validate: { presence: true },
               full_name: N_("Hostname facts"),
               description: N_("List of facts to use for the hostname (first wins)")
 
-            validates "discovery_hostname", presence: true
-
             setting "discovery_auto",
               type: :boolean,
               default: false,
@@ -95,11 +94,10 @@ class Engine < ::Rails::Engine
             setting "discovery_prefix",
               type: :string,
               default: "mac",
+              validate: { presence: true },
               full_name: N_("Hostname prefix"),
               description: N_("The default prefix to use for the host name, must start with a letter")
 
-            validates "discovery_prefix", presence: true
-
             setting "discovery_fact_column",
               type: :array,
               default: [],