Update rubocop-performance requirement from 1.19.1 to 1.20.1 (#372)

Updates the requirements on [rubocop-performance](https://github.com/rubocop/rubocop-performance) to permit the latest version.
- [Release notes](https://github.com/rubocop/rubocop-performance/releases)
- [Changelog](https://github.com/rubocop/rubocop-performance/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop-performance@v1.19.1...v1.20.1)

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

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

Gemfile

@@ -6,6 +6,6 @@ gemspec
 
 gem 'coveralls', require: false
 gem 'rubocop', '1.57.2', require: false
-gem 'rubocop-performance', '1.19.1', require: false
+gem 'rubocop-performance', '1.20.1', require: false
 gem 'rubocop-rake', '0.6.0', require: false
 gem 'rubocop-rspec', '2.25.0', require: false