Use of unknown feature gates should be a deny-by-default lint

[jethrogb]

#52644 made it an error to specify a #![feature(...)] that the compiler doesn't know about. It could be a deny-by-default lint instead. @varkor in #52644 (comment):

I don't see a strong reason not to make the unknown feature error a deny-by-default lint instead, which would allow you to silence it.

[Centril]

I think this should be a warning instead which makes it more feasible to actually garbage collect old feature gates.

[tmandry]

This is really important for doing soft migrations across compiler versions. For instance:

1. An existing feature foo is used in the code base
2. That feature gets split into foo and min_foo
3. The codebase owner has no way of atomically updating the compiler and code at the same time

In this case you'd need #![allow(unknown_features)], so you can also add #![feature(min_foo)] before updating the compiler and completing the migration.