Remove the ignore rule for chat_names. integration_id column

What?

The column integration_id was ignored in %15.10, so according to the documentation we should drop the column in %16.0

Proposal

1. Drop the column with the following migration:

# frozen_string_literal: true

class RemoveChatNamesIntegrationIdColumn < Gitlab::Database::Migration[2.1]
  enable_lock_retries!

  def change
    remove_column :chat_names, :integration_id, :integer
  end
end

2. Remove the following code from app/models/chat_name.rb

include IgnorableColumns
ignore_column :integration_id, remove_with: '16.0', remove_after: '2023-04-22'

3. Update the following specs:

• ee/spec/services/integrations/slack_option_service_spec.rb
• spec/db/schema_spec.rb