Builds: use .NET SDK 2.2.3xx (#1192)

Build images have slipped again, testing a rev to latest stable SDK generation and manually installing on Windows since Appveyor doesn't have it.

appveyor.yml

@@ -6,6 +6,7 @@ init:
   - git config --global core.autocrlf input
 
 install:
+- cmd: choco install dotnetcore-sdk --version 2.2.301
 - cmd: >-
     cd tests\RedisConfigs\3.0.503
 

global.json

 
 {
   "sdk": {
-    "version": "2.1.500"
+    "version": "2.2.300"
   }
 }
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