[Bug #13671] Fix that "ss" in look-behind causes syntax error

Fixes k-takata/Onigmo#92.

This fix was ported from oniguruma:
kkos/oniguruma@257082d

k-takata/Onigmo@b1a5445

Author: k-takata

language/regexp_spec.rb

@@ -112,7 +112,7 @@
     /foo.(?<=\d)/.match("fooA foo1").to_a.should == ["foo1"]
   end
 
-  ruby_bug "#13671", ""..."3.6" do # https://bugs.ruby-lang.org/issues/13671
+  ruby_bug "#13671", ""..."3.5" do # https://bugs.ruby-lang.org/issues/13671
     it "handles a lookbehind with ss characters" do
       r =  Regexp.new("(?<!dss)", Regexp::IGNORECASE)
       r.should =~ "✨"