Reintroduce warnings postponed in 6.0

A few warnings were introduced near the 6.0 release, so we can't comply with the "2 versions minimum with warnings", so for 6.0 these warnings were suppressed in https://github.com/pytest-dev/pytest/pull/7362.

We should reintroduce them in 6.1.