Update global.json to use .NET 9.0.0 stable instead of RC 1 (#6846)

* Initial plan

* Update .NET 9 RC 1 references to stable 9.0.0 in global.json

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

global.json

@@ -7,11 +7,11 @@
     "runtimes": {
       "dotnet": [
         "8.0.0",
-        "9.0.0-rc.1.24431.7"
+        "9.0.0"
       ],
       "aspnetcore": [
         "8.0.0",
-        "9.0.0-rc.1.24452.1"
+        "9.0.0"
       ]
     }
   },