Increase default elastic agent long poll duration to ~30 minutes

[pchila]

• Depends on Increase default long poll duration to ~10 minutes #2544

In issue #2257 it was requested to increase the poll duration to 35 minutes, however since merging elastic/fleet-server#2491 it's up to the client (elastic agent) to present the poll timeout in the checkin request body.
To ensure a progressive rollout of the feature the PR #2408 introduces a long poll default duration of 12 minutes (so that fleet-server answers in ~ 10 mins) for version 8.8

Starting from 8.9 we would like to increase that default duration to ~30 minutes as originally specified in #2257

/cc @cmacknz

[cmacknz]

Closing as not planned.