Remove rubocop extensions suggestions

When we run rubocop, it will raise a tip regaring suggested exentions.
Those extensions are not useful for us, so we disabled the tip.

Result after running rubocop:
```ruby
Tip: Based on detected gems, the following RuboCop extension libraries might be helpful:
  * rubocop-rake (https://rubygems.org/gems/rubocop-rake)
  * rubocop-rspec (https://rubygems.org/gems/rubocop-rspec)

You can opt out of this message by adding the following to your config (see https://docs.rubocop.org/rubocop/extensions.html#extension-suggestions for more options):
  AllCops:
    SuggestExtensions: false
```

.rubocop.yml

@@ -2,6 +2,7 @@ inherit_from: .rubocop_todo.yml
 
 AllCops:
   NewCops: enable
+  SuggestExtensions: false
 
 Naming/FileName:
  Exclude: