fix: require C++20 for windows plugin build

[Predidit]

Recent MSVC toolchains, such as the one shipped with Visual Studio 2026 / MSVC 14.51, now reject the deprecated experimental coroutine path used by older C++/WinRT builds.

Without explicitly enabling C++20, the Flutter Windows template default can compile the plugin as C++17, which causes C++/WinRT headers to fall back to <experimental/coroutine> and fail with:

STL1011: The /await compiler option, <experimental/coroutine>,
<experimental/generator>, and <experimental/resumable> are deprecated
by Microsoft and will be REMOVED SOON.