fix: Return :commit since :ack does not commit in v2 (#84)

## Checklist

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- [x] Code conforms to the [Elixir
Styleguide](https://github.com/christopheradams/elixir_style_guide)

## Problem

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When using the `brod_group_subscriber_v2` our `handle_message/3`
callback was returning `{:ok, :ack, state}`. This was the old way to
commit messages; in v2, it only acks and doesn't commit based on this
comment:

https://github.com/kafka4beam/brod/blob/master/src/brod_group_subscriber_worker.erl#L85

## Details

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Changed to commit

lib/kafee/consumer/brod_worker.ex

@@ -27,7 +27,7 @@ defmodule Kafee.Consumer.BrodWorker do
 
   @doc false
   @impl :brod_group_subscriber_v2
-  @spec handle_message(:brod.message(), map()) :: {:ok, :ack, map()}
+  @spec handle_message(:brod.message(), map()) :: {:ok, :commit, map()}
   def handle_message(
         message,
         %{
@@ -51,6 +51,6 @@ defmodule Kafee.Consumer.BrodWorker do
       headers: message[:headers]
     })
 
-    {:ok, :ack, state}
+    {:ok, :commit, state}
   end
 end