Update minitest requirement from ~> 5.27 to ~> 6.0

Updates the requirements on [minitest](https://github.com/minitest/minitest) to permit the latest version.
- [Changelog](https://github.com/minitest/minitest/blob/master/History.rdoc)
- [Commits](minitest/minitest@v5.27.0...v6.0.1)

---
updated-dependencies:
- dependency-name: minitest
  dependency-version: 6.0.1
  dependency-type: direct:production
...

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

Author: dependabot[bot]

Gemfile

@@ -8,7 +8,7 @@ gemspec
 gem 'appraisal'
 gem 'byebug'
 gem 'm'
-gem 'minitest', '~> 5.27' # TODO: relax when the minimum required Rails version will include rails/rails@99395e1ea401acbc23d4f6b2a8657cdb82f921bd
+gem 'minitest', '~> 6.0' # TODO: relax when the minimum required Rails version will include rails/rails@99395e1ea401acbc23d4f6b2a8657cdb82f921bd
 gem 'mocha'
 gem 'rake'
 gem 'rubocop'