Suppress RuboCop extension suggestions (#484)

This PR suppresses the following information:

```console
$ bundle exec rubocop
(snip)

Tip: Based on detected gems, the following RuboCop extension libraries might be helpful:
  * rubocop-i18n (https://rubygems.org/gems/rubocop-i18n)

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

There's no need to introduce a third-party RuboCop extension unless there's a specific need.

.rubocop.yml

@@ -7,6 +7,7 @@ plugins:
 
 AllCops:
   TargetRubyVersion: 2.7
+  SuggestExtensions: false
 
 Gemspec/DevelopmentDependencies:
   Enabled: true