onnxruntime

Update MAUI model tester tool to .net8 (#19907)

### Description
<!-- Describe your changes. -->
Update to .net8. Didn't want to build with the latest VS2022 using net6
(which was EOL last year).



### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->