Demote warning (#6080)

This can happen under heavy load - no value in warning here.

Scenario this happens:

- New slot
- We get asked about what fork to build on
- We start building (create inherent gets called)
- We learn about a better fork (but Babe does not care as it already has a fork)
- 6 seconds passed - New slot
- Babe asks about what for to build on - we answer with the best block we learned about 6 seconds ago (slightly after we got asked the last time)
- We start building (on that old block)
- Milliseconds later we learn about a new block (the one from the slot we are actually in now)
- We kick the old leaf -> create inherent fails

Full discussion: https://github.com/paritytech/substrate/issues/12375