[Security Solution][Detections][PIT] - Update `findPreviousThresholdSignals` to use PIT

[yctercero]

In an effort to reduce the number of expensive queries in Kiana, core is tracking efforts to move to the use of PIT rather than search queries that pull for 10,000 items.

See Kibana core ticket here.

File
https://github.com/elastic/kibana/blob/main/x-pack/plugins/security_solution/server/lib/detection_engine/signals/threshold/find_previous_threshold_signals.ts#L82

[elasticmachine]

Pinging @elastic/security-detections-response (Team:Detections and Resp)

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)

[marshallmain]

The core ticket mentions looking for queries against Saved Objects, and the previous threshold signals are in alerts indices rather than Saved Objects - does that mean we can continue to use 10k here?

[yctercero]

We can close this out as this ticket was indeed specific to querying against SOs.