Use pwsh shorthand consistently in WebGPU test stage
Convert all PowerShell steps in plugin-win-webgpu-test-stage.yml to the - pwsh: shorthand for consistency. The Python test step previously used the PowerShell@2 task with pwsh: true; the NuGet test steps used the - powershell: shorthand which runs Windows PowerShell 5.1. All now run under PowerShell 7+.
