Update Mapster.Tool to include .net7.0 as target framework

[janiskulj]

When trying to run dotnet mapster model (version '8.3.0') in .net 7 project I'm getting:

Could not load file or assembly 'System.Runtime, Version=7.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. The system cannot find the file specified.

I think it is simmilar issue as #390

[andrerav]

Have you tried the prerelease version?

[janiskulj]

It works in prerelease version 8.4.0-pre06
Thank you