Removed nuget username from appveyor config.

Author: sipsorcery

Diff for: .appveyor.yml

@@ -64,7 +64,6 @@ deploy:
   - provider: NuGet
     server: https://nuget.pkg.github.com/sipsorcery/index.json
     artifact: /.*(\.|\.s)nupkg/
-    username: sipsorcery
     api_key:
       secure: E58r+OknoQn8+bsPRT6l3U2K4kfOpDiGCo1C75LkVg+R/RBHpY//J8UCXEfVvyRB
     on:

Diff for: global.json

@@ -0,0 +1,6 @@
+{
+  "sdk": {
+    "version": "6.0.100",
+    "rollForward": "latestMajor"
+  }
+}