Bump .NET in CI to 8.x (#3683)

This is the current LTS release. Based on an [n=1
experiment](https://github.com/pulumi/pulumi-azure-native/actions/runs/11689711913/job/32554028746),
it also helps with the OOM on the pulumi/examples test.

.github/actions/install/action.yml

@@ -37,7 +37,7 @@ runs:
       if: inputs.skip_dotnet_and_java != 'true'
       uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: 6.x
+        dotnet-version: 8.x
 
     - name: Setup Java
       if: inputs.skip_dotnet_and_java != 'true'