Update rubocop-minitest requirement from = 0.8.0 to = 0.9.0 (#21)

Updates the requirements on [rubocop-minitest](https://github.com/rubocop-hq/rubocop-minitest) to permit the latest version.
- [Release notes](https://github.com/rubocop-hq/rubocop-minitest/releases)
- [Changelog](https://github.com/rubocop-hq/rubocop-minitest/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop-minitest@v0.8.0...v0.9.0)

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

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

example.gemspec

@@ -35,6 +35,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "minitest-reporters", "~> 1.3"
   spec.add_development_dependency "rake", "~> 13.0"
   spec.add_development_dependency "rubocop", "0.81.0"
-  spec.add_development_dependency "rubocop-minitest", "0.8.0"
+  spec.add_development_dependency "rubocop-minitest", "0.9.0"
   spec.add_development_dependency "rubocop-performance", "1.5.2"
 end