Update webrick requirement from ~> 1.8.1 to ~> 1.9.1

Updates the requirements on [webrick](https://github.com/ruby/webrick) to permit the latest version.
- [Release notes](https://github.com/ruby/webrick/releases)
- [Commits](ruby/webrick@v1.8.1...v1.9.1)

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

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

govuk_design_system_formbuilder.gemspec

@@ -49,6 +49,6 @@ Gem::Specification.new do |s|
   s.add_development_dependency("sassc", "~> 2.4.0")
   s.add_development_dependency("slim", "~> 5.2.0")
   s.add_development_dependency("slim_lint", "~> 0.31.0")
-  s.add_development_dependency("webrick", "~> 1.8.1")
+  s.add_development_dependency("webrick", "~> 1.9.1")
   s.add_development_dependency("redcarpet", "~> 3.6.0")
 end