Update rubocop-minitest requirement from 0.20.1 to 0.21.0 (#172)

Updates the requirements on [rubocop-minitest](https://github.com/rubocop/rubocop-minitest) to permit the latest version.
- [Release notes](https://github.com/rubocop/rubocop-minitest/releases)
- [Changelog](https://github.com/rubocop/rubocop-minitest/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop-minitest@v0.20.1...v0.21.0)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Gemfile

@@ -7,7 +7,7 @@ gem "minitest-reporters", "~> 1.1"
 gem "mocha", "~> 1.11"
 gem "rake", "~> 13.0"
 gem "rubocop", "1.32.0"
-gem "rubocop-minitest", "0.20.1"
+gem "rubocop-minitest", "0.21.0"
 gem "rubocop-packaging", "0.5.1"
 gem "rubocop-performance", "1.14.3"
 gem "rubocop-rake", "0.6.0"