The change was made in .NET 5, not 6. In .NET Core 3.1 the Console log format was fixed. In .NET 5 this is now customizable with 3 predefined formatters: Simple (the old one), Systemd and Json (the default). It's possible to create a custom formatter.

As the docs show, it's possible to use the Simple formatter by using the AddSimpleConsole method instead of AddConsole:

    using ILoggerFactory loggerFactory =
        LoggerFactory.Create(builder =>
            builder.AddSimpleConsole(options =>
            {
                options.IncludeScopes = true;
                options.SingleLine = true;
                options.TimestampFormat = "hh:mm:ss ";
            }));

There are similar methods for the other two formatters: AddSystemdConsole and AddJsonConsole.

It's possible to set the formatter through configuration :

{
    "Logging": {
        "LogLevel": {
            "Default": "Information",
            "Microsoft": "Warning",
            "Microsoft.Hosting.Lifetime": "Information"
        },
        "Console": {
            "LogLevel": {
                "Default": "Information",
                "Microsoft": "Warning",
                "Microsoft.Hosting.Lifetime": "Information"
            },
            "FormatterName": "json",
            "FormatterOptions": {
                "SingleLine": true,
                "IncludeScopes": true,
                "TimestampFormat": "HH:mm:ss ",
                "UseUtcTimestamp": true,
                "JsonWriterOptions": {
                    "Indented": true
                }
            }
        }
    },
    "AllowedHosts": "*"
}

Finally it's possible to create a completely new formatter by inheriting from ConsoleFormatter and overriding the Write method :

public sealed class CustomFormatter : ConsoleFormatter, IDisposable
{
   ...

    public override void Write<TState>(
        in LogEntry<TState> logEntry,
        IExternalScopeProvider scopeProvider,
        TextWriter textWriter)
    {
        string? message =
            logEntry.Formatter?.Invoke(
                logEntry.State, logEntry.Exception);

        if (message is null)
        {
            return;
        }

        CustomLogicGoesHere(textWriter);
        textWriter.WriteLine(message);
    }

   ...
}