Update rubocop requirement from >= 1.25, < 1.56 to >= 1.25, < 1.57

Updates the requirements on [rubocop](https://github.com/rubocop/rubocop) to permit the latest version.
- [Release notes](https://github.com/rubocop/rubocop/releases)
- [Changelog](https://github.com/rubocop/rubocop/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop@v1.25.0...v1.28.2)

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

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

radius-spec.gemspec

@@ -32,7 +32,7 @@ Gem::Specification.new do |spec|
   spec.required_ruby_version = ">= 2.5" # rubocop:disable Gemspec/RequiredRubyVersion
 
   spec.add_runtime_dependency "rspec", "~> 3.7"
-  spec.add_runtime_dependency "rubocop", ">= 1.25", "< 1.56"
+  spec.add_runtime_dependency "rubocop", ">= 1.25", "< 1.57"
   spec.add_runtime_dependency "rubocop-rails", ">= 2.13", "< 2.21"
 
   spec.add_development_dependency "bundler", ">= 2.2.10"