Update rubocop-performance requirement from ~> 1.20.0 to ~> 1.21.0

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@v1.20.0...v1.21.0)

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

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

voxpupuli-rubocop.gemspec

@@ -16,7 +16,7 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency 'rake', '~> 13.0', '>= 13.0.6'
   s.add_runtime_dependency 'rubocop', '~> 1.63.5'
   s.add_runtime_dependency 'rubocop-minitest', '~> 0.35.0'
-  s.add_runtime_dependency 'rubocop-performance', '~> 1.20.0'
+  s.add_runtime_dependency 'rubocop-performance', '~> 1.21.0'
   s.add_runtime_dependency 'rubocop-rake', '~> 0.6.0'
   s.add_runtime_dependency 'rubocop-rspec', '~> 2.26'
 end