TreeEnsembleRegressor is deprecated in ONNX

**System Information (please complete the following information):**
- OS & Version: Windows 11
- ML.NET Version: 5.0.0
- .NET Version: 10

**Describe the bug**

As a workaround for #6942 , I converted my ML models to ONNX

Upon inspection of the generated ONNX files , I discovered that they use TreeEnsembleRegressor operator which, as per the onnx documentation, [it's been deprecated in favour of the new TreeEnsemble operator.](https://onnx.ai/onnx/operators/onnx_aionnxml_TreeEnsembleRegressor.html#summary)

This means that any ONNX model currently being exported will stop working some time in the future when the deprecated operators are removed from the code.

**To Reproduce**
- Create a project using regression trees and export a ONNX model
- Inspect the ONNX model with Netron and check that it contains a TreeEnsembleRegressor operator.

**Expected behavior**

TreeEnsemble operator should be used instead of TreeEnsembleRegressor, for future, long term compatibility.

**Screenshots, Code, Sample Projects**