Add dotnet 8 to copilot setup steps

Author: benbp

.github/workflows/copilot-setup-steps.yml

@@ -17,7 +17,9 @@ jobs:
 
       - uses: actions/setup-dotnet@v4
         with:
-          dotnet-version: 9.0.x
+          dotnet-version: |
+            8.0.x
+            9.0.x
 
       - name: Build azsdk cli
         run: |