Use the same Windows image on both VS and MinGW tasks

Hi,

The VS and MinGW Windows images are merged on Andres' pg-vm-images
repository now [1]. So, the old pg-ci-windows-ci-vs-2019 and
pg-ci-windows-ci-mingw64 images will not be updated from now on. This
new merged image (pg-ci-windows-ci) needs to be used on both VS and
MinGW tasks.
I attached a patch for using pg-ci-windows-ci Windows image on VS and
MinGW tasks.

CI run when pg-ci-windows-ci is used:
https://cirrus-ci.com/build/6063036847357952

[1]: https://github.com/anarazel/pg-vm-images/commit/6747f676b97348d47f041b05aa9b36cde43c33fe

Regards,
Nazir Bilal Yavuz
Microsoft