doc: develop: gsg: move from ubuntu 20.04 to 22.04 as minimum version

Ubuntu 20.04 has entered "security fixes only" mode since May 2025, so
we can simplify the getting started guide by making the instructions
apply to 22.04 and onwards, and remove the instructions for adding
Kitware APT repository in the process.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>