Update rubocop requirement from 1.56.4 to 1.57.2

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@v1.56.4...v1.57.2)

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

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

Gemfile

@@ -6,7 +6,7 @@ gem "minitest", "~> 5.11"
 gem "minitest-ci", "~> 3.4"
 gem "minitest-reporters", "~> 1.3"
 gem "rake", "~> 13.0"
-gem "rubocop", "1.56.4"
+gem "rubocop", "1.57.2"
 gem "rubocop-md", "1.2.0"
 gem "rubocop-minitest", "0.32.2"
 gem "rubocop-packaging", "0.5.2"