Update slim_lint requirement from ~> 0.28.0 to ~> 0.31.0

Updates the requirements on [slim_lint](https://github.com/sds/slim-lint) to permit the latest version.
- [Release notes](https://github.com/sds/slim-lint/releases)
- [Changelog](https://github.com/sds/slim-lint/blob/main/CHANGELOG.md)
- [Commits](sds/slim-lint@v0.28.0...v0.31.0)

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

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

govuk_design_system_formbuilder.gemspec

@@ -48,7 +48,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency("sass")
   s.add_development_dependency("sassc", "~> 2.4.0")
   s.add_development_dependency("slim", "~> 5.2.0")
-  s.add_development_dependency("slim_lint", "~> 0.28.0")
+  s.add_development_dependency("slim_lint", "~> 0.31.0")
   s.add_development_dependency("webrick", "~> 1.8.1")
   s.add_development_dependency("redcarpet", "~> 3.6.0")
 end