Update nanoc requirement from ~> 4.12.15 to ~> 4.13.0

Updates the requirements on [nanoc](https://github.com/nanoc/nanoc) to permit the latest version.
- [Release notes](https://github.com/nanoc/nanoc/releases)
- [Changelog](https://github.com/nanoc/nanoc/blob/main/nanoc/NEWS.md)
- [Commits](nanoc/nanoc@nanoc-cli-v4.12.15...nanoc-cli-v4.13.0)

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

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

govuk_design_system_formbuilder.gemspec

@@ -42,7 +42,7 @@ Gem::Specification.new do |s|
 
   # Required for the guide
   s.add_development_dependency("htmlbeautifier", "~> 1.4.1")
-  s.add_development_dependency("nanoc", "~> 4.12.15")
+  s.add_development_dependency("nanoc", "~> 4.13.0")
   s.add_development_dependency("rouge", "~> 4.2.0")
   s.add_development_dependency("rubypants", "~> 0.7.0")
   s.add_development_dependency("sass")