Multiple Invariant[Monoid] instances

As @floating-cat pointed out here in Gitter, there are currently two Invariant[Monoid] instances in Cats. One is inside the Invariant companion object. Another is the InvariantSemigroupal[Monoid] instance that is located here. In practice, this seems unlikely to cause issues to me (since if you ask for an Invariant[Monoid] the former should "win"). However, it does seem to be an oversight.

If we were to remove the instance from the Invariant companion object, then people would need to start using an import (such as cats.implicits._) to pick up the Invariant[Monoid] instance, which they wouldn't have previously needed to do. Removing it would also be binary incompatible. I'm not sure whether or not it's worth changing this, but if so, I think that we'd have to wait for Cats 3 to make this change.

Compare the Invariant companion object with core/src/main/scala/cats/instances/invariant.scala to confirm the duplicate Invariant[Monoid] instances and their resolution behavior. Review the stated import and binary-compatibility concerns; done requires a documented decision about whether this belongs in a future Cats 3 change.