I guess one of your packages depends on 9.0 version
you can check the project.assets.json
Thank you very much for pointing us on the right direction. It was a recently updated indirect dependency that indeed had a dependency on System.Text.Json 9.0.0.
I think you updated the version by mistake, you can check your project file to make sure all abp packages are 8.3.4 and Microsoft packages are 8.0.x.
PS, Leptonx theme version should be 8.3.4
And try dotnet clean&abp clean to clean the solution.