Set target version for Performance/UnfreezeString

As stated in the class documentation, unary plus operator is available
from Ruby 2.3.

This fix prevent RuboCop Performance to raise an offense when configured
to support Ruby 2.2

changelog/fix_target_version_for_unfreeze_string_cop.md

@@ -0,0 +1 @@
+* [#373](https://github.com/rubocop/rubocop-performance/pull/373): Set target version for `Performance/UnfreezeString`. ([@tagliala][])

lib/rubocop/cop/performance/unfreeze_string.rb

@@ -26,6 +26,9 @@ module Performance
       #   +''
       class UnfreezeString < Base
         extend AutoCorrector
+        extend TargetRubyVersion
+
+        minimum_target_ruby_version 2.3
 
         MSG = 'Use unary plus to get an unfrozen string literal.'
         RESTRICT_ON_SEND = %i[dup new].freeze

spec/rubocop/cop/performance/unfreeze_string_spec.rb

@@ -112,4 +112,12 @@
       String.new(capacity: 100)
     RUBY
   end
+
+  context 'when Ruby <= 2.2', :ruby22 do
+    it 'does not register an offense for an empty string with `.dup`' do
+      expect_no_offenses(<<~RUBY)
+        "".dup
+      RUBY
+    end
+  end
 end