Fix deprecation warning about Supervisor.terminate_child with PID

[colinsmetz]

Hello!

When using KafkaEx with Elixir 1.11, this warning appears very frequently:

warning: Supervisor.terminate_child/2 with a PID is deprecated, please use DynamicSupervisor instead
  (elixir 1.11.2) lib/supervisor.ex:857: Supervisor.terminate_child/2
  (stdlib 3.14) gen_server.erl:727: :gen_server.try_terminate/3
  (stdlib 3.14) gen_server.erl:912: :gen_server.terminate/10
  (stdlib 3.14) proc_lib.erl:226: :proc_lib.init_p_do_apply/3

This PR removes those deprecation warnings by applying the fix suggested in the message.

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[jbruggem]

(disclosure: Colin is a direct colleague if mine).

[jbruggem]

Given that the change is trivial and that all the tests passed, I'll go ahead and merge this.