Revert "fix builder to install dotnet 8 instead of 9"

This reverts commit 881b0c4.

Author: ericsink

.github/workflows/dotnet.yml

@@ -24,7 +24,7 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: 8.0.x
+        dotnet-version: 9.0.x
     - name: Install workload ios
       run: dotnet workload install ios
     - name: Install workload maccatalyst