Increase tap slack thread_lookback_days

[pnadolny13]

Related to https://github.com/meltano/administration/issues/436 we'll want to make sure we extract as much slack thread data as we can within the 90 day window. Right now we set thread_lookback_days to 1 day but we should consider making it 90 days. I'm not sure how this setting works so I'll need to dig into the implementation to see if we can precisely replicate the threads with replies and ideally only the new messages within a thread.