Fix deprecations for Ruby 2.7.0 in Waterdrop 2.0

[mensfeld]

.........../home/mencio/Software/Github/karafka/waterdrop/lib/water_drop/producer/buffer.rb:95: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
/home/mencio/Software/Github/karafka/waterdrop/.bundle/ruby/2.7.0/bundler/gems/rdkafka-ruby-d07c9595a063/lib/rdkafka/producer.rb:61: warning: The called method `produce' is defined here
..................................................................................................................................................................................../home/mencio/Software/Github/karafka/waterdrop/lib/water_drop/producer/async.rb:24: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
/home/mencio/Software/Github/karafka/waterdrop/.bundle/ruby/2.7.0/bundler/gems/rdkafka-ruby-d07c9595a063/lib/rdkafka/producer.rb:61: warning: The called method `produce' is defined here
...../home/mencio/Software/Github/karafka/waterdrop/lib/water_drop/producer/async.rb:46: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
/home/mencio/Software/Github/karafka/waterdrop/.bundle/ruby/2.7.0/bundler/gems/rdkafka-ruby-d07c9595a063/lib/rdkafka/producer.rb:61: warning: The called method `produce' is defined here
....................................................../home/mencio/Software/Github/karafka/waterdrop/lib/water_drop/producer/sync.rb:28: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call