Robot Name: clarify what "unique" means

The specification for the [Robot Names exercise](https://github.com/exercism/problem-specifications/blob/main/exercises/robot-name/description.md) says the solution "must ensure that every *existing* robot has a *unique* name" (confusing words emphasized).

The use of "existing" together with "unique" makes "unique" unclear. This could mean either:

1. "ensure that the name of every existing robot is different from the name of every other existing robot."
2. or: "ensure that the name of every existing robot is unique among all robot names ever used."

Meeting specification 2 will necessarily meet specification 1, but the opposite isn't true. Since one can imagine a situation in which it would be okay to reuse a name that was used in the past as long as it's not currently in use, it's not obvious that we should simply meet specification 2. So I think the problem specification should be clarified.