Update slim_lint requirement from ~> 0.22.0 to ~> 0.24.0

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/master/CHANGELOG.md)
- [Commits](sds/slim-lint@v0.22.0...v0.24.0)

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

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

govuk-components.gemspec

@@ -44,6 +44,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency("sass")
   spec.add_development_dependency("sassc", "~> 2.4.0")
   spec.add_development_dependency("slim", "~> 4.1.0")
-  spec.add_development_dependency("slim_lint", "~> 0.22.0")
+  spec.add_development_dependency("slim_lint", "~> 0.24.0")
   spec.add_development_dependency("webrick", "~> 1.8.1")
 end