Update puma requirement from ~> 5.6 to >= 5.6, < 7.0

Updates the requirements on [puma](https://github.com/puma/puma) to permit the latest version.
- [Release notes](https://github.com/puma/puma/releases)
- [Changelog](https://github.com/puma/puma/blob/master/History.md)
- [Commits](puma/puma@v5.6.0...v6.0.0)

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

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

govuk_app_config.gemspec

@@ -23,7 +23,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency "logstasher", "~> 2.1"
   spec.add_dependency "plek", "~> 4"
   spec.add_dependency "prometheus_exporter", "~> 2.0"
-  spec.add_dependency "puma", "~> 5.6"
+  spec.add_dependency "puma", ">= 5.6", "< 7.0"
   spec.add_dependency "rack-proxy", "~> 0.7"
   spec.add_dependency "sentry-rails", "~> 5.3"
   spec.add_dependency "sentry-ruby", "~> 5.3"