FIX: Revert Demon::DemonBase back to Demon::Base (#8132)

I introduced DemonBase because I had got some conflict between `demon/base.rb` and `jobs/base.rb`, however, to not rename base class, it is possible to use regex on absolute path in Zeitwerk custom inflector.

script/demon_test/parent.rb

@@ -4,7 +4,7 @@ require File.expand_path("../../../config/environment", __FILE__)
 
 puts "Parent is now loaded"
 
-class ForkExecDemon < Demon::DemonBase
+class ForkExecDemon < Demon::Base
   def self.prefix
     "fork-exec-child"
   end