Support Ruby 3.0.0 keyword arguments interop

Ruby 3.0.0 keywords arguments do not permit Hash <-> keyword arguments interop. Instead, we should add a double asterisk operator(`**`) for it. Signed-off-by: Hiroshi Hatake <cosmo0920.oucc@gmail.com>

Author: cosmo0920

lib/fluent/plugin/out_kafka.rb

@@ -215,7 +215,7 @@ def emit(tag, es, chain)
  chain.next
 
  # out_kafka is mainly for testing so don't need the performance unlike out_kafka_buffered.
- producer = @kafka.producer(@producer_opts)
+ producer = @kafka.producer(**@producer_opts)
 
  es.each do |time, record|
  if @output_include_time

lib/fluent/plugin/out_kafka_buffered.rb

@@ -239,7 +239,7 @@ def get_producer
  @producers_mutex.synchronize {
  producer = @producers[Thread.current.object_id]
  unless producer
- producer = @kafka.producer(@producer_opts)
+ producer = @kafka.producer(**@producer_opts)
  @producers[Thread.current.object_id] = producer
  end
  producer