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.

[jiegillet]

Reopening the issue, because some of the links will need to be updated when 1.18 comes out:

• https://hexdocs.pm/elixir/case-cond-and-if.html#if-unless => https://hexdocs.pm/elixir/case-cond-and-if.html#if (probably)
• https://elixirschool.com/en/lessons/basics/control-structures/#if-and-unless-0 => ?

The link checker probably won't help us catching these link changes, since the changes are in the hash part.