Update minitest requirement from ~> 5.0 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@v5.0.0...v6.0.0)

---
updated-dependencies:
- dependency-name: minitest
  dependency-version: 6.0.0
  dependency-type: direct:development
...

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

Author: dependabot[bot]

emailable.gemspec

@@ -30,7 +30,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'bundler'
   s.add_development_dependency 'rake', '~> 13.0'
   s.add_development_dependency 'amazing_print'
-  s.add_development_dependency 'minitest', '~> 5.0'
+  s.add_development_dependency 'minitest', '~> 6.0'
   s.add_development_dependency 'minitest-reporters'
   s.add_development_dependency 'activemodel'
   s.add_development_dependency 'rubocop-cache-ventures'