TreeEnsembleRegressor is deprecated in ONNX

[vpenades]

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.

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