Upgrade CI's mingw to some recent version

Currently CI is using mingw 8.1.0, it's public package was released on 2018/05/24 (time from SourceForge), which is quite old nowadays, and it is preventing developers from using new features such as lack of modern DPI functions introduced in Windows 10. Since Qt6's minimum supported OS is Windows 10 1809, many pre-win10 code have been removed in dev branch already, so I think it's safe to drop old mingw support.

I suggest upgrade CI's mingw to some recent version such as 9.0 or 10.0, or just use llvm-mingw alone. I personally prefer use llvm-mingw alone because it compiles much faster and it's packages are always a lot newer than the original mingw-w64 project.