Update mdl requirement from ~> 0.11.0 to ~> 0.13.0

Updates the requirements on [mdl](https://github.com/mivok/markdownlint) to permit the latest version.
- [Changelog](https://github.com/markdownlint/markdownlint/blob/main/CHANGELOG.md)
- [Commits](markdownlint/markdownlint@v0.11.0...v0.13.0)

---
updated-dependencies:
- dependency-name: mdl
  dependency-version: 0.13.0
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Gemfile

@@ -20,7 +20,7 @@ end
 group :development do
   gem 'json_spec', '~> 1.1', '>= 1.1.5'
 
-  gem 'mdl', '~> 0.11.0'
+  gem 'mdl', '~> 0.13.0'
 
   gem 'mocha', '~> 2.7'