According to Microsoft, this can be resolved by adding the following to your csproj.
<PackageReference Include="Microsoft.NETCore.Targets" Version="3.0.0" PrivateAssets="all" />
https://learn.microsoft.com/en-us/nuget/reference/errors-and-warnings/nu1605#issue-1
“Certain combinations of packages which shipped with .NET Core 1.0 and 1.1 are not compatible with each other when they are referenced together in a .NET Core 3.0 or higher project, and a RuntimeIdentifier is specified. The problematic packages generally start with System. or Microsoft., and have version numbers between 4.0.0 and 4.3.1. In this case, the downgrade message will have a package starting with runtime. in the dependency chain.”