Change default value for RemoveLoggerFactoryFilter to true

[snakefoot]

See also #482

[snakefoot]

One might be surprised when doing this:

   ILogger<Program> logger = LoggerFactory.Create(builder => {
          builder.SetMinimumLevel(LogLevel.Warning);  // Explicit LogLevel will not apply to NLog
          builder.AddNLog();
        }).CreateLogger<Program>();
    logger.LogInformation("Hello World!");

[snakefoot]

One could consider changing RemoveLoggerFactoryFilter from a boolean into LoggerFactoryMinimumLevel and when LogLevel.Trace, then it would be the same as RemoveLoggerFactoryFilter = true. And when assigned to (LogLevel?)null then it would be the same as RemoveLoggerFactoryFilter = false.

Thern this pull-request would be changing default-value for LoggerFactoryMinimumLevel from (LogLevel?)null to LogLevel.Trace.