SQL Server cannot create assembly as it references system.runtime, version=5.0.0.0

I'm trying to create my first CLR assembly. I have created a dll in VS and now I am trying to use it in SQL Server.

I try to create assembly with this code:

CREATE ASSEMBLY SQLCLRroy
FROM '...\SqlExternalFunctions\bin\Debug\net5.0\SQLExternalFunctions.dll';

but I get an error

Assembly 'SqlExternalFunctions' references assembly 'system.runtime, version=5.0.0.0, culture=neutral, publickeytoken=b03f5f7f11d50a3a.', which is not present in the current database.

After trawling the net I try this:

CREATE ASSEMBLY [System.Runtime] 
FROM 'C:\Program Files\dotnet\packs\Microsoft.NETCore.App.Ref\5.0.0\ref\net5.0\System.Runtime.dll'

which again results in an error

Warning: The Microsoft .NET Framework assembly 'system.runtime, version=5.0.0.0, culture=neutral, publickeytoken=b03f5f7f11d50a3a.' you are registering is not fully tested in the SQL Server hosted environment and is not supported.

Do I need to stop my .NET code from referencing System.Runtime somehow? The only package I have is System.Data.SqlClient

My .NET code is very simple:

using System;
using System.Linq;

public static class SqlExternalFunctions
{
    [Microsoft.SqlServer.Server.SqlFunction]
    public static int myTest(int param )
    {
        return param;
    }
}

Use .Net Framework not .Net Core as the Target Framework in VS