I suspect this is because all nuget versions remain .net 4.8 only - this is the latest info i have seen about it, Rhino.Inside .NET Core 7.0 - #7 by curtisw you can also see an example of the warning you may have in that thread
Congrats on getting your plugin going for .NET 7! The Grasshopper warning because we are currently only distributing the .NET 4.8 version in the nuget packages (it’s the same one you are ignoring during the build using <NoWarn>NU1701</NoWarn>, and is completely harmless.
We will eventually include net7.0 assemblies in our nuget packages which will address that.