[PATCH rtems-docs v2] Update package list for Ubuntu

[Anders Montonen]

Hi,

> On 20 Dec 2021, at 17:22, Ryan Long <ryan.long at oarcorp.com> wrote:
> 
> Updated the latest version referenced of Ubuntu to 21.04, updated
> package list, and identified what packages need to still be installed
> afterwards to pass sb-check.

This should be 20.04, to match the patch

> -The latest version is Ubuntu 18.04.1 LTS 64-bit. This section also includes
> +The latest version is Ubuntu 20.04.3 LTS 64-bit. This section also includes

Regards,
Anders Montonen