Update rubocop requirement from ~> 1.26.0 to ~> 1.59.0

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](https://github.com/rubocop/rubocop/compare/v1.26.0...v1.59.0)

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

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

wpscan.gemspec

@@ -27,7 +27,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rake',                '~> 13.0'
   s.add_development_dependency 'rspec',               '~> 3.12.0'
   s.add_development_dependency 'rspec-its',           '~> 1.3.0'
-  s.add_development_dependency 'rubocop',             '~> 1.26.0'
+  s.add_development_dependency 'rubocop',             '~> 1.59.0'
   s.add_development_dependency 'rubocop-performance', '~> 1.13.0'
   s.add_development_dependency 'simplecov',           '~> 0.21.0'
   s.add_development_dependency 'simplecov-lcov',      '~> 0.8.0'