[Ingest Manager] Better default for agent checkin long polling

[nchaulet]

Description

Currently the default value for the agent checkin long polling timeout is 1 minute.
Increasing this will improve a lot the performance of fleet, I tried with 5 minute and it seems great. (Check with cloud potential issue here)

Also we should probably use #73730 to set the socketTimeout for the checkin route.

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)