Use Thor for built-in secret task

Currently we use both Thor and Rake for `bin/rails` commands.
We eventually want to get all the built-ins task promoted to Thor Commands.
This migrates the secret task to Thor.

railties/lib/rails/commands/secret/secret_command.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+module Rails
+  module Command
+    class SecretCommand < Base # :nodoc:
+      desc "secret", "Generate a cryptographically secure secret key (this is typically used to generate a secret for cookie sessions)."
+      def perform
+        require "securerandom"
+        puts SecureRandom.hex(64)
+      end
+    end
+  end
+end

railties/lib/rails/tasks/misc.rake

@@ -1,11 +1,5 @@
 # frozen_string_literal: true
 
-desc "Generate a cryptographically secure secret key (this is typically used to generate a secret for cookie sessions)."
-task :secret do
-  require "securerandom"
-  puts SecureRandom.hex(64)
-end
-
 desc "List versions of all Rails frameworks and the environment"
 task about: :environment do
   puts Rails::Info