Updates the plan limit default for ai flow_schedules

What does this MR do and why?

Update Ai::FlowSchedules default plan limit from 10 to 100. See original MR implementation.

I'd initially set the default to 10 per project matching the precedent in the Ci::PipelineSchedule, however, now that we're releasing the scheduled trigger internally and we want teams to be able to set as much schedules as possible during the experimentation phase, whilst also being mindful of resource management.

See thread discussion.

Since it's internal, shall we start with 50 and see where that takes us.

We're going with 100 instead of the suggested 50 to prevent hard blockage during teams' experimental efforts.

Related to #594180

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