Update rubocop requirement from 1.50.2 to 1.56.1 (#437)

Updates the requirements on [rubocop](https://github.com/rubocop/rubocop) to permit the latest version.
- [Release notes](https://github.com/rubocop/rubocop/releases)
- [Changelog](https://github.com/rubocop/rubocop/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop@v1.50.2...v1.56.1)

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

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

Gemfile

@@ -15,7 +15,7 @@ gem 'appraisal'
 gem 'overcommit', '0.60.0'
 
 # Pin tool versions (which are executed by Overcommit) for CI builds
-gem 'rubocop', '1.50.2' # The last version that supports Ruby 2.6
+gem 'rubocop', '1.56.1' # The last version that supports Ruby 2.6
 
 gem 'simplecov', '~> 0.22.0'
 gem 'simplecov-lcov', '~> 0.8.0'