Upgrade to a 6.0 SDK

- without this, generated assemblies are unusable in aspnetcore after that brands to 6.0.0
- in other words, this is needed for dotnet/aspnetcore#24983 to test correctly

Author: dougbu

global.json

@@ -1,6 +1,6 @@
 {
   "tools": {
-    "dotnet": "5.0.100-rc.1.20452.10",
+    "dotnet": "6.0.100-alpha.1.20472.11",
     "runtimes": {
       "dotnet": [
         "3.1.8"
@@ -11,7 +11,7 @@
     }
   },
   "sdk": {
-    "version": "5.0.100-rc.1.20452.10",
+    "version": "6.0.100-alpha.1.20472.11",
     "allowPrerelease": true,
     "rollForward": "latestMajor"
   },