AppVeyor: VS2019 image, skip tags, reduce clone depth

Using VS2019 should hopefully "just work".
Clone depth only needs to be big enough to handle all queued builds.
Don't build tags, just results in duplicate builds on master branch.

Author: angularsen

appveyor.yml

@@ -1,12 +1,13 @@
 version: '{build}'
-image: Visual Studio 2017
+image: Visual Studio 2019
+skip_tags: true
 skip_commits:
  files:
    - docs/*
    - '**/*.md'
 
 # Only fetch the latest commits to reduce time to download on agent, but support pushes containing multiple commits
-clone_depth: 50
+clone_depth: 20
 
 # build cache to preserve files/folders between builds
 cache: