Redundant allow-newer warnings

## Summary

When upgrading to a new version of a compiler, it's normal to add a bunch of `allow-newer`s to one's cabal.project file. Over time these grow to be redundant as packages raise their upper bounds.

I then find myself iteratively going through my cabal.project file and testing whether each of these is still needed. This is quite labour intensive.

## Suggestion

Add a warning if an `allow-newer` is no longer needed. Ideally this would point out the line in the cabal.project file where the override is specified. Then I could just react to these warnings when updating my index-state and I wouldn't have to manually check.