Allow Puma 6

Puma 6 was disallowed in 2022, because back then Rails itself generated
Gemfiles with `'puma', "< 6". This is not the case anymore, in fact,
Rails comes with Rack 3 which instead is incompatible with Puma 5.

Gemfile

@@ -29,7 +29,7 @@ gem 'simplecov-cobertura', require: false
 gem 'rack', '< 3', require: false
 gem 'rake', require: false, groups: [:lint, :release]
 gem 'rails-controller-testing', require: false
-gem 'puma', '< 6', require: false
+gem 'puma', '< 7', require: false
 gem 'i18n-tasks', '~> 0.9', require: false
 gem 'rspec_junit_formatter', require: false
 gem 'yard', require: false