Update simplecov requirement from ~> 0.20.0 to ~> 0.21.2 (#58)

Updates the requirements on [simplecov](https://github.com/simplecov-ruby/simplecov) to permit the latest version.
- [Release notes](https://github.com/simplecov-ruby/simplecov/releases)
- [Changelog](https://github.com/simplecov-ruby/simplecov/blob/main/CHANGELOG.md)
- [Commits](simplecov-ruby/simplecov@v0.20.0...v0.21.2)

Signed-off-by: dependabot[bot] <[email protected]>

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

Author: dependabot[bot]

Gemfile

@@ -8,5 +8,5 @@ gemspec
 gem 'minitest', '~> 5.0'
 gem 'rake', '~> 13.0'
 gem 'rubocop', '~> 1.11.0'
-gem 'simplecov', '~> 0.20.0', require: false # CodeClimate not compatible with 0.18+ yet - https://github.com/codeclimate/test-reporter/issues/413
+gem 'simplecov', '~> 0.21.2', require: false # CodeClimate not compatible with 0.18+ yet - https://github.com/codeclimate/test-reporter/issues/413
 gem 'simplecov-lcov', '< 0.8'