Add .simplecov to rubocop default include list

[robotdana]

This list looks fairly comprehensive, so lets add .simplecov the ruby config file for simplecov gem

https://github.com/colszowka/simplecov#using-simplecov-for-centralized-config

[koic]

Can you add the chengelog entry and update the following testcase?

diff --git a/spec/rubocop/target_finder_spec.rb b/spec/rubocop/target_finder_spec.rb
index f21311654..64ccc1187 100644
--- a/spec/rubocop/target_finder_spec.rb
+++ b/spec/rubocop/target_finder_spec.rb
@@ -36,6 +36,7 @@ RSpec.describe RuboCop::TargetFinder, :isolated_environment do

   RUBY_FILENAMES = %w[.irbrc
                       .pryrc
+                      .simplecov
                       Appraisals
                       Berksfile
                       Brewfile

Ref: #7715

[bbatsov]

Thanks!