chore: downgrade rubocop and rubocop-shopify versions to match the code quality verifier

rubocop-nexusmods.gemspec

@@ -25,6 +25,6 @@ Gem::Specification.new do |spec|
     end
   end
 
-  spec.add_dependency "rubocop", "~> 1.51"
-  spec.add_dependency "rubocop-shopify", "~> 2.15"
+  spec.add_dependency "rubocop", "1.29.1"
+  spec.add_dependency "rubocop-shopify", "2.6.0"
 end