Update MSYS2 default installed package to it's recommendation.

[wy-luke]

In MSYS2 official doc, the default installed package became mingw-w64-ucrt-x86_64-gcc. It says that ucrt is a better solution.

UCRT (Universal C Runtime) is a newer version which is also used by Microsoft Visual Studio by default. It should work and behave as if the code was compiled with MSVC.

So I thought maybe it's better to change our doc according to this change.

I changed the mingw-w64-x86_64-toolchain to mingw-w64-ucrt-x86_64-toolchain and changed the path from C:\msys64\mingw64\bin to C:\msys64\ucrt64\bin and so on.

[AlexandraKemperMS]

This looks good! Thanks for catching this change and helping keep our docs up to date!

[gregvanl]

Adding don't merge label while the C++ extension engineering team has a review.

[gregvanl]

Thank you again @wy-luke for this suggestion.
Now that the C++ extension supports UCRT, the team has submitted #6631 which covers this PR