Update build.yml

Author: pwelter34

.github/workflows/build.yml

@@ -41,10 +41,7 @@ jobs:
       - name: Setup .NET
         uses: actions/setup-dotnet@v4
         with:
-          dotnet-version: |
-              3.1.x
-              6.0.x
-              8.0.x
+          dotnet-version: 9.0.x
 
       - name: Restore Dependencies
         run: dotnet restore