update target framework to dotnet6.0 (#15615)
### Description
Upgrade dotnet E2E test target framework to dotnet6.0
### Motivation and Context
Fix dotnet3.1 deprecation issue which broke nuget building pipeline.
The error message in NuGet_Test_Linux_CPU was
```
To install missing framework, download:
https://aka.ms/dotnet-core-applaunch?framework=Microsoft.NETCore.App&framework_version=3.1.0&arch=x64&rid=ubuntu.20.04-x64
. Please check the diagnostic logs for more information.
```
Test Run:
https://dev.azure.com/aiinfra/Lotus/_build/results?buildId=300655&view=results.
