Bump rake version 13 or higher to suppress ostruct warnings

This commit bumps the required Rake version to suppress the following warnings:

```ruby
$ ruby -v
ruby 3.4.0dev (2024-10-21T16:48:53Z master 5131fb5dbe) +PRISM [x86_64-linux]
$ bundle exec rake 2>&1 |grep -i ostruct
ostruct was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0.
You can add ostruct to your Gemfile or gemspec to silence this warning.
/home/yahonda/.rbenv/versions/trunk/lib/ruby/gems/3.4.0+0/gems/rake-12.3.3/lib/rake/rake_test_loader.rb:2: warning: ostruct was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0.
You can add ostruct to your Gemfile or gemspec to silence this warning.
./home/yahonda/src/github.com/rails/sprockets/lib/rake/sprocketstask.rb:2: warning: ostruct was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0.
You can add ostruct to your Gemfile or gemspec to silence this warning.
```

- Rake v13.2.0 does not depend on OpenStruct
https://github.com/ruby/rake/releases/tag/v13.2.0
ruby/rake#545

- Ruby 3.4.0dev emits OpenStruct warnings
https://bugs.ruby-lang.org/issues/20309
ruby/ruby#10428

Author: yahonda

sprockets.gemspec

@@ -29,7 +29,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency "minitest", "~> 5.0"
   s.add_development_dependency "nokogiri", "~> 1.3"
   s.add_development_dependency "rack-test", "~> 2.0.0"
-  s.add_development_dependency "rake", "~> 12.0"
+  s.add_development_dependency "rake", "~> 13.2"
   s.add_development_dependency "sass", "~> 3.4"
   s.add_development_dependency "sassc", "~> 2.0"
   s.add_development_dependency "uglifier", ">= 2.3"