Warn user if we encounter too many tasks groups on the scheduler

[hendrikmakait]

Problem

As described in #8677, having too many task groups can become prohibitively expensive, in particular for the TaskProgress dashboard.

Solution

We should warn the user if there are too many tasks groups on the scheduler. This might either be a simple threshold as mentioned in #8677 or something that also involves a ratio of tasks to task groups.

[fjetter]

With #8681 I hope that we won't see any more issues popping up. Having "many" task groups is still something unusual but I'm not sure how bad it really is