What is the right way to fix error NU1605: Detected package downgrade - log4net

Unfortunately, there's no other option than the package owner to upgrade their references or you adding the clausule in the csproj. Please see this GitHub issue for more information: https://github.com/dotnet/core/issues/907

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://docs.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."