doc/develop/getting_started: update for Ubuntu 22.04

In Ubuntu 22.04 there is no need to add any extra repositories
to meet the minimum required versions for tools like cmake and dtc.
In fact, the Kitware repos don't even work in Ubuntu 22.04.

So, instructions updated to explain that step is only required for
Ubuntu versions older than 22.04.

Signed-off-by: Diego Herranz <diegoherranz@diegoherranz.com>