Fix build when targeting Windows 7 as platform.

[mol123]

This change makes more of the code introduced in https://github.com/yhirose/cpp-httplib/pull/1775 conditional on feature macros.

CreateFile2, CreateFileMappingFromApp and MapViewOfFileFromApp are available only starting from Windows 8.

• https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfile2
• https://learn.microsoft.com/en-us/windows/win32/api/memoryapi/nf-memoryapi-createfilemappingfromapp
• https://learn.microsoft.com/en-us/windows/win32/api/memoryapi/nf-memoryapi-mapviewoffilefromapp