Update rake requirement from ~> 12.3.1 to >= 12.3.1, < 13.1.0

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](https://github.com/ruby/rake/compare/v12.3.1...v13.0.0

)

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

just-the-docs.gemspec

@@ -15,7 +15,7 @@ Gem::Specification.new do |spec|
 
   spec.add_runtime_dependency "jekyll", "~> 3.8.5"
   spec.add_runtime_dependency "jekyll-seo-tag", "~> 2.0"
-  spec.add_runtime_dependency "rake", "~> 12.3.1"
+  spec.add_runtime_dependency "rake", ">= 12.3.1", "< 13.1.0"
 
   spec.add_development_dependency "bundler", "~> 2.0.1"
 end