fix: update to supported appveyor image

sheurich · jsha · commit cb7f033d0a4f · 2024-02-20T14:37:11.000-08:00
diff --git a/appveyor.yml b/appveyor.yml
@@ -1,4 +1,4 @@
-image: Visual Studio 2017
+image: Visual Studio 2019
 
 hosts:
   example.letsencrypt.org: 127.0.0.1
