Update redis requirement from < 5 to < 6

Updates the requirements on [redis](https://github.com/redis/redis-rb) to permit the latest version.
- [Changelog](https://github.com/redis/redis-rb/blob/master/CHANGELOG.md)
- [Commits](redis/redis-rb@v0.2.0...v4.8.1)

---
updated-dependencies:
- dependency-name: redis
  dependency-type: direct:production
...

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

govuk_sidekiq.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |spec|
 
   spec.add_dependency "gds-api-adapters", ">= 19.1.0"
   spec.add_dependency "govuk_app_config", ">= 1.1"
-  spec.add_dependency "redis", "< 5"
+  spec.add_dependency "redis", "< 6"
   spec.add_dependency "redis-namespace", "~> 1.6"
   spec.add_dependency "sidekiq", "~> 6.5", ">= 6.5.12"