Hide milestone confusing message to the user

This PR change the log level when a user load a plugin with a milestone
option to debug instead of warn.

Fixes: #4562

Fixes #4603

logstash-core/lib/logstash/config/mixin.rb

@@ -147,7 +147,7 @@ def plugin_status(status = nil)
     # inside the gemspec.
     def milestone(m = nil)
       @logger = Cabin::Channel.get(LogStash)
-      @logger.warn(I18n.t('logstash.plugin.deprecated_milestone', :plugin => config_name))
+      @logger.debug(I18n.t('logstash.plugin.deprecated_milestone', :plugin => config_name))
     end
 
     # Define a new configuration setting

logstash-core/spec/logstash/plugin_spec.rb

@@ -111,7 +111,7 @@ class LogStash::Filters::MyTestFilter < LogStash::Filters::Base
 
 
     it 'logs a warning if the plugin use the milestone option' do
-      expect_any_instance_of(Cabin::Channel).to receive(:warn)
+      expect_any_instance_of(Cabin::Channel).to receive(:debug)
         .with(/stromae plugin is using the 'milestone' method/)
 
       class LogStash::Filters::Stromae < LogStash::Filters::Base