Do not propagate cancellation to the upstream in Flow flat* operators

[qwwdfsad]

A new mental model is now perfectly aligned with the rest of the coroutines:

• Where one [semantically] expects a new child to be launched (e.g. in merge or in flatMap*), the cancellation works in one direction as expected -- cancellation of the child is not propagated to the parent. It's especially important when the "virtual child" (flow returned by flat*) can have a different cancellation scope (e.g. it can be a separate UI fragment)
• Where everything happens in-place, e.g. flattenConcat, the behaviour stays the same

Fixes #2942

[dkhalanskyjb]

The docs for flattenConcat still state that it's just an optimized flattenMerge(concurrency = 1), but they now have different downstream cancellation behavior. I didn't check the other operators to see if their docs also need to be updated.

[qwwdfsad]

I've updated the doc.

flatMapMerge and flattenMerge indeed now have an observable difference in cancellation behaviour for concurrency equal 1 and > 1. This is a deliberate design decision as the fact that concurrency == 1 is optimized into sequential transformation. Sequential -> no child coroutines -> in place invocations -> in-place cancellation is propagated

[dkhalanskyjb]

Yeah, I didn't have issues with the change here, only with the fact that the documentation would now be misleading.