Deprecate unless in Elixir 1.18

[angelikatyborska]

In Elixir 1.18, unless is being soft-deprecated. We should remove it from our syllabus and from a few of the example solutions.

[jiegillet]

Sounds good.

We also have an analyzer rule about avoiding unless with else. I suppose we won't need it once 1.18 is out, but until then I guess we should leave it.