Doctest available on Kino:0.8 but not yet available Fly.io

You can use ` {:kino, "~> 0.8.0"}` (with `Elixir: 1.14`) locally and `Doctest` works ex nihilo but Fly.io only accepts ` {:kino, "~> 0.7.0"}` for the moment.

The Fly.io machine is running an older version of `Elixir`:
```
Operating System: Linux
CPU Information: AMD EPYC
Number of Available Cores: 1
Available memory: 221.14 MB
Elixir 1.13.2
Erlang 24.1.7
```

With 0.8, Doctests are executed automatically when the module is compiled. Nice!
https://news.livebook.dev/whats-new-in-livebook-0.8-4gQiEE