Use RuboCop Base class not Cop

- This was showing a deprecation warning with the latest RuboCop version.

```
warning: Inheriting from `RuboCop::Cop::Cop` is deprecated. Use `RuboCop::Cop::Base` instead.
For more information, see https://docs.rubocop.org/rubocop/v1_upgrade_notes.html
``

Author: issyl0

lib/rubocop/cop/graphql/heredoc.rb

@@ -5,7 +5,7 @@ module RuboCop
   module Cop
     module GraphQL
       # Public: Cop for enforcing non-interpolated GRAPHQL heredocs.
-      class Heredoc < Cop
+      class Heredoc < Base
         def on_dstr(node)
           check_str(node)
         end

lib/rubocop/cop/graphql/overfetch.rb

@@ -8,7 +8,7 @@ module RuboCop
   module Cop
     module GraphQL
       # Public: Rubocop for catching overfetched fields in ERB templates.
-      class Overfetch < Cop
+      class Overfetch < Base
         if defined?(RangeHelp)
           # rubocop 0.53 moved the #source_range method into this module
           include RangeHelp