mingw builds are going to start failing with Ruby 3.4

[gjtorikian]

I've been looking into why commonmarker's mingw builds have been failing:

 thread 'main' panicked at C:\Users\runneradmin/.cargo\registry\src\index.crates.io-6f17d22bba15001f\rb-sys-0.9.103\build/main.rs:49:6:
  generate bindings: ClangDiagnostic("D:/rubyinstaller-head-x64/include/ruby-3.4.0+1/ruby/internal/stdckdint.h:48:11: error: 'stdckdint.h' file not found with <angled> include; use \"quotes\" instead\n")

stckdint.h is a C23 header: ruby/ruby#10632

Clang 18 supported it: https://releases.llvm.org/18.1.6/tools/clang/docs/ReleaseNotes.html#c23-feature-support

But the mingw builds explicitly lock to v15:

actions/setup-ruby-and-rust/action.yml

Lines 108 to 109 in 83d0264

	# for now we have to use clang-15 for mingw builds, since our bindgen is too old to work with clang-16
	mingw: clang-15

I couldn't go back far enough to understand why the mingw builds couldn't upgrade. But, this will almost certainly break for others later this month when Ruby 3.4 is released.

cc @ianks

[ianks]

we can try to bump to clang 18, but IIRC there were other issues