Update KoreBuild channel

korebuild.json

 {
-  "$schema": "https://raw.githubusercontent.com/aspnet/BuildTools/dev/tools/korebuild.schema.json",
-  "channel": "dev"
+  "$schema": "https://raw.githubusercontent.com/aspnet/BuildTools/release/2.1/tools/korebuild.schema.json",
+  "channel": "release/2.1"
 }