Tweak new cop template

Follow #9421.

This PR tweaks new cop template.

New cop development will rarely need `subject(:cop) { described_class.new(config) }`.
So I think there aren't any development tips lost by replacing it with
`RSpec.describe ..., :config`.

Perhaps `let(:config) { RuboCop::Config.new }` can also be removed,
but I'll look at it separately. Maybe not as explicitly unnecessary
as `subject(:cop) { described_class.new(config) }`.

lib/rubocop/cop/generator.rb

@@ -81,9 +81,7 @@ def on_send(node)
       SPEC_TEMPLATE = <<~SPEC
         # frozen_string_literal: true
 
-        RSpec.describe RuboCop::Cop::%<department>s::%<cop_name>s do
-          subject(:cop) { described_class.new(config) }
-
+        RSpec.describe RuboCop::Cop::%<department>s::%<cop_name>s, :config do
           let(:config) { RuboCop::Config.new }
 
           # TODO: Write test code

spec/rubocop/cop/generator_spec.rb

@@ -110,9 +110,7 @@ def on_send(node)
       generated_source = <<~SPEC
         # frozen_string_literal: true
 
-        RSpec.describe RuboCop::Cop::Style::FakeCop do
-          subject(:cop) { described_class.new(config) }
-
+        RSpec.describe RuboCop::Cop::Style::FakeCop, :config do
           let(:config) { RuboCop::Config.new }
 
           # TODO: Write test code