Use ::Struct to avoid collisions with cop namespace

We were trying to add a custom cop called `Struct/RequiresTypedStructHelper`, but ran into a problem where we got an error like:

```
undefined method `new' for RuboCop::Cop::Struct:Module
/Users/technicalpickles/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rubocop-rails-2.21.1/lib/rubocop/cop/mixin/index_method.rb:105:in `module:IndexMethod'
/Users/technicalpickles/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rubocop-rails-2.21.1/lib/rubocop/cop/mixin/index_method.rb:6:in `module:Cop'
/Users/technicalpickles/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rubocop-rails-2.21.1/lib/rubocop/cop/mixin/index_method.rb:4:in `module:RuboCop'
/Users/technicalpickles/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rubocop-rails-2.21.1/lib/rubocop/cop/mixin/index_method.rb:3:in `top (required)'
/Users/technicalpickles/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rubocop-rails-2.21.1/lib/rubocop/cop/rails_cops.rb:7:in `require_relative'
/Users/technicalpickles/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rubocop-rails-2.21.1/lib/rubocop/cop/rails_cops.rb:7:in `top (required)'
/Users/technicalpickles/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/rubocop-rails-2.21.1/lib/rubocop-rails.rb:16:in `require_relative'
```

It turned out this module has method that were calling `Struct.new`, but Ruby
constant lookup ended up using our `Rubocop::Cop::Struct`, instead of the actual
`Struct` class.

We were able to work around it by changing the constant name, but
`Struct` is always going to be part of the stdlib so a possible name
collision.

lib/rubocop/cop/mixin/index_method.rb

@@ -102,7 +102,7 @@ def execute_correction(corrector, node, correction)
       end
 
       # Internal helper class to hold match data
-      Captures = Struct.new(
+      Captures = ::Struct.new(
         :transformed_argname,
         :transforming_body_expr
       ) do
@@ -112,7 +112,7 @@ def noop_transformation?
       end
 
       # Internal helper class to hold autocorrect data
-      Autocorrection = Struct.new(:match, :block_node, :leading, :trailing) do
+      Autocorrection = ::Struct.new(:match, :block_node, :leading, :trailing) do
         def self.from_each_with_object(node, match)
           new(match, node, 0, 0)
         end