Support Elixir ~> 1.17

[ziopio]

In Elixir version 1.17, the function elixir:start_iex/0 has been removed.
This makes the grisp.ini boot args, suggested in the README, only valid up to elixir 1.16.
We need to document this and find an alternative function to start the elixir shell.

[ziopio]

This seams to be the relevant commit

elixir-lang/elixir@b3710de#diff-4ef990cb3eea7c6f679e231c209b8e72b628b95318756d6620883131ae084b1c