Switch Appveyor CI to VS2019 stable image

The current appveyor config is using the VS2019 preview image so the latest prebuilt Qt5.12.11 binaries can be used, see bitcoin#22224.

Appveyor updated the Visual Studio 2019 image to msbuild v16.10.1 on 14th of June. This is the version used to build the latest Qt binaries and removes the need to use the Appveyor VS2019 preview image.

.appveyor.yml

@@ -1,6 +1,6 @@
 version: '{branch}.{build}'
 skip_tags: true
-image: Visual Studio 2019 Preview
+image: Visual Studio 2019
 configuration: Release
 platform: x64
 clone_depth: 5