Skip spec which does not work on mingw

* https://github.com/ruby/ruby/runs/1994688198
  ruby 3.1.0dev (2021-02-27T16:42:17Z master 8305a48413) [x64-mingw32]
  C-API Thread function rb_thread_call_without_gvl runs a C function with the global lock unlocked and unlocks IO with the generic RUBY_UBF_IO
  Example took longer than the configured timeout of 60.0s

spec/ruby/optional/capi/thread_spec.rb

@@ -155,7 +155,7 @@ describe "C-API Thread function" do
       end
     end
 
-    guard_not -> { platform_is :mingw and ruby_version_is ""..."2.7" } do
+    platform_is_not :mingw do
       it "runs a C function with the global lock unlocked and unlocks IO with the generic RUBY_UBF_IO" do
         thr = Thread.new do
           @t.rb_thread_call_without_gvl_with_ubf_io