Automerge: [Github] Add Timeouts to Premerge Jobs (#164038)

The premerge jobs currently use the default timeouts. If something ends
up hanging, they run for the full six hours. Given our jobs never
actually take six hours other than when something hangs, we should set
the timeouts lower to not use up machine capacity when something does go
wrong. These limits were currently chosen conservatively, being a decent
amount above the maximum run times I have seen over the past couple
months, excluding six hour timeouts.

Author: boomanaiden154