Update rake requirement from ~> 12.0 to ~> 13.2

Updates the requirements on [rake](https://github.com/ruby/rake) to permit the latest version.
- [Release notes](https://github.com/ruby/rake/releases)
- [Changelog](https://github.com/ruby/rake/blob/master/History.rdoc)
- [Commits](ruby/rake@v12.0.0...v13.2.1)

---
updated-dependencies:
- dependency-name: rake
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

artwork-template.gemspec

@@ -15,5 +15,5 @@ Gem::Specification.new do |spec|
   spec.add_runtime_dependency "jekyll", "~> 3.9"
 
   spec.add_development_dependency "bundler", "~> 1.16"
-  spec.add_development_dependency "rake", "~> 12.0"
+  spec.add_development_dependency "rake", "~> 13.2"
 end