[Fix #12714] Fix an error for Gemspec/RequiredRubyVersion

Fixes #12714.

This PR fixes an error for `Gemspec/RequiredRubyVersion`
when `required_ruby_version` is specified with `Gem::Requirement.new`
and is higher than `TargetRubyVersion`.

When specifying only an upper version limit, an offense is registered
because lower Ruby version that matches `TargetRubyVersion` is unknown.

changelog/fix_an_error_for_gemspec_required_ruby_version.md

@@ -0,0 +1 @@
+* [#12714](https://github.com/rubocop/rubocop/issues/12714): Fix an error for `Gemspec/RequiredRubyVersion` when `required_ruby_version` is specified with `Gem::Requirement.new` and is higher than `TargetRubyVersion`. ([@koic][])

lib/rubocop/cop/gemspec/required_ruby_version.rb

@@ -110,6 +110,8 @@ def extract_ruby_version(required_ruby_version)
             end
           end
 
+          return unless required_ruby_version
+
           required_ruby_version.str_content.scan(/\d/).first(2).join('.')
         end
 

spec/rubocop/cop/gemspec/required_ruby_version_spec.rb

@@ -40,6 +40,15 @@
       RUBY
     end
 
+    it 'registers an offense when `required_ruby_version` is specified with `Gem::Requirement.new` and is higher than `TargetRubyVersion`' do
+      expect_offense(<<~RUBY)
+        Gem::Specification.new do |spec|
+          spec.required_ruby_version = Gem::Requirement.new('< 3.4')
+                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `required_ruby_version` and `TargetRubyVersion` (2.7, which may be specified in .rubocop.yml) should be equal.
+        end
+      RUBY
+    end
+
     it 'recognizes a Gem::Requirement with multiple requirements and does not register an offense' do
       expect_no_offenses(<<~RUBY)
         Gem::Specification.new do |spec|