use new event api in generator filter template

Fixes #5355

lib/pluginmanager/templates/filter-plugin/lib/logstash/filters/example.rb.erb

@@ -34,7 +34,7 @@ class LogStash::Filters::<%= classify(plugin_name) %> < LogStash::Filters::Base
     if @message
       # Replace the event message with our message as configured in the
       # config file.
-      event["message"] = @message
+      event.set("message", @message)
     end
 
     # filter_matched should go in the last line of our successful code

lib/pluginmanager/templates/filter-plugin/spec/filters/example_spec.rb.erb

@@ -15,7 +15,7 @@ describe LogStash::Filters::<%= classify(plugin_name) %> do
 
     sample("message" => "some text") do
       expect(subject).to include("message")
-      expect(subject['message']).to eq('Hello World')
+      expect(subject.get('message')).to eq('Hello World')
     end
   end
 end