Update rubocop-performance requirement from = 1.20.2 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@v1.20.2...v1.21.0)

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

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

panolint-ruby.gemspec

@@ -20,6 +20,6 @@ Gem::Specification.new do |spec|
   # Pinned to latest versions and will be updated by Dependabot
   # so that code bases using panolint-ruby will get the latest versions
   spec.add_dependency "rubocop", "1.63.1"
-  spec.add_dependency "rubocop-performance", "1.20.2"
+  spec.add_dependency "rubocop-performance", "1.21.0"
   spec.add_dependency "rubocop-rspec", "2.29.1"
 end