Remove .NET Core 3.1 and add .NET 7/8

**Is your feature request related to a problem? Please describe.**
.NET Core 3.1 reach End of Support in December, while it is available latest .NET 7 version and upcoming .NET 8

**Describe the solution you'd like**
Upgrade the projects

**Describe alternatives you've considered**
N/A

**Additional context**
Must wait masesgroup/JCOReflector#118