V8 Add retainedFileTimeLimit to serilog configuration

[PBHansen]

Serilog supports the option to add a retained file time limit to a rolling log file which determines how long it should be retained (using TimeSpan).

https://github.com/serilog/serilog-sinks-file/blob/764bb4a655b006fd59f1274c74d54d51a0dde79e/src/Serilog.Sinks.File/FileLoggerConfigurationExtensions.cs#L237

This could be quite useful if one wanted to retain log files for x months. My old solution was to keep x log files, but sometimes Umbraco creates several files on the same day, even if rolling interval is set to Day.