Compatible .NETCore versions

Tried using the action with a project that runs on .NET 5, but I keep getting
```
The framework 'Microsoft.NETCore.App', version '3.1.2' was not found.
- The following frameworks were found:
5.0.11 at [C:\Users\runneradmin\AppData\Local\Microsoft\dotnet\shared\Microsoft.NETCore.App]
```
This is the link to the failed workflow: https://github.com/PC-Chair-Fight/project-backend/runs/4132660755?check_suite_focus=true

Start with the failed workflow linked in the issue and inspect which runtime version the action requires. Reproduce the reported .NET 5 case; done when the action runs without requesting Microsoft.NETCore.App 3.1.2.

由索引模型根据 Issue 内容生成。