engSys, redirect autorest deprecation message from stderr to stdout

[XiaofeiCao]

Description

autorest 3.8.0 will emit deprecation message to stderr with exit code 0, which will break CI that's using it.

This PR redirects stderr to stdout. Alternative be, pinning autorest to previous 3.7.2, as we likely not going to upgrade it.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[weidongxu-microsoft]

We've tried failOnStderr=false in autorest.java repo pipeline (it should be false by default anyway)
https://learn.microsoft.com/en-us/azure/devops/pipelines/tasks/reference/powershell-v2?view=azure-pipelines

It does not work.

[Copilot]

Pull request overview

This PR updates the code-generation verification script to avoid CI failures caused by AutoRest 3.8.0 emitting a deprecation message to stderr while still exiting with code 0.

Changes:

• Redirects the Swagger regeneration script’s error stream (stderr / stream 2) into the success output stream when capturing output.