exa is unmaintained, and advices to use the fork eza instead

`exa` (in section "System Utilities") is unmaintained, and advices on its [webpage](https://github.com/ogham/exa) to use the fork `eza` instead:
[https://github.com/eza-community/eza](https://github.com/eza-community/eza)
