Move VS 2019 builds from AppVeyor to GH Actions

GitHub runners unfortunately do not come with MinGW or VS 2010
pre-installed, so moving those builds will be more involved.  MinGW-w64
is not a good replacement for MinGW as it is far more complete.

This gives at least some feedback for all supported platforms via the
GitHub Actions system.

(cherry picked from commit 4e788fc6dc)

.appveyor.yml

@@ -1,6 +1,5 @@
 image:
     - Visual Studio 2015
-    - Visual Studio 2019
 branches:
     only:
         - ci
@@ -21,26 +20,13 @@ environment:
         - GENERATOR: Visual Studio 10 2010
           BUILD_SHARED_LIBS: OFF
           CFLAGS: /WX
-        - GENERATOR: Visual Studio 16 2019
-          BUILD_SHARED_LIBS: ON
-          CFLAGS: /WX
-        - GENERATOR: Visual Studio 16 2019
-          BUILD_SHARED_LIBS: OFF
-          CFLAGS: /WX
 matrix:
     fast_finish: true
-    exclude:
-        - image: Visual Studio 2015
-          GENERATOR: Visual Studio 16 2019
-        - image: Visual Studio 2019
-          GENERATOR: Visual Studio 10 2010
-        - image: Visual Studio 2019
-          GENERATOR: MinGW Makefiles
 for:
 -
     matrix:
-        except:
-            - GENERATOR: Visual Studio 10 2010
+        only:
+            - GENERATOR: MinGW Makefiles
     build_script:
         - set PATH=%PATH:C:\Program Files\Git\usr\bin=C:\MinGW\bin%
         - cmake -S . -B build -G "%GENERATOR%" -DBUILD_SHARED_LIBS=%BUILD_SHARED_LIBS%