Update json-schema requirement from >= 2.8, < 4.2 to >= 2.8, < 4.3

Updates the requirements on [json-schema](https://github.com/voxpupuli/json-schema) to permit the latest version.
- [Changelog](https://github.com/voxpupuli/json-schema/blob/master/CHANGELOG.md)
- [Commits](voxpupuli/json-schema@v2.8.0...v4.2.0)

---
updated-dependencies:
- dependency-name: json-schema
  dependency-type: direct:production
...

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

govuk_schemas.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = %w[lib]
 
   # This should be kept in sync with the json-schema version of publishing-api.
-  spec.add_dependency "json-schema", ">= 2.8", "< 4.2"
+  spec.add_dependency "json-schema", ">= 2.8", "< 4.3"
 
   spec.add_development_dependency "climate_control"
   spec.add_development_dependency "pry-byebug"