Remove downlevel runtimes from global.json (#40393)

* Remove downlevel runtimes from global.json

* Update global.json

global.json

@@ -5,15 +5,11 @@
   "tools": {
     "dotnet": "7.0.100-preview.3.22163.1",
     "runtimes": {
-      "dotnet": [
-        "2.1.30",
-        "$(MicrosoftNETCoreBrowserDebugHostTransportVersion)"
-      ],
       "dotnet/x86": [
         "$(MicrosoftNETCoreBrowserDebugHostTransportVersion)"
       ],
-      "aspnetcore": [
-        "3.1.21"
+      "dotnet": [
+        "$(MicrosoftNETCoreBrowserDebugHostTransportVersion)"
       ]
     },
     "Git": "2.22.0",