On a net5.0 build packagemanagement picks an idosyncratic targetframework

[KevinRansom]

run fsi
Enter #r "nuget:FSharp.Data";;

Open the project file created for package management:
[Loading C:\Users\codec\AppData\Local\Temp\nuget\9416--2a7b0245-67ff-41a1-8e7e-01d96d3ddaf4\Project.fsproj.fsx]

Observe netcoreapp5.0

  <PropertyGroup>
    <TargetFramework>netcoreapp5.0</TargetFramework>
    <RuntimeIdentifier>win-x64</RuntimeIdentifier>
    <IsPackable>false</IsPackable>