Exclude x64-mingw32 from the list of supported platforms

Because rake-compiler 1.5.0 does not support x64-mingw32 with recent
Ruby versions, and they use x64-mingw-ucrt instead.

Author: kateinoigakukun

.github/workflows/build-gems.yml

@@ -18,7 +18,7 @@ jobs:
         id: fetch
         with:
           supported-ruby-platforms: |
-            exclude: [arm-linux]
+            exclude: [arm-linux, x64-mingw32]
           stable-ruby-versions: |
             exclude: [head]