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>

doc/develop/getting_started/index.rst

@@ -22,7 +22,7 @@ Click the operating system you are using.
 
    .. group-tab:: Ubuntu
 
-      This guide covers Ubuntu version 20.04 LTS and later.
+      This guide covers Ubuntu version 22.04 LTS and later.
       If you are using a different Linux distribution see :ref:`installation_linux`.
 
       .. code-block:: bash
@@ -73,19 +73,6 @@ The current minimum required version for the main dependencies are:
 
       .. _install_dependencies_ubuntu:
 
-      #. If using an Ubuntu version older than 22.04, it is necessary to add extra
-         repositories to meet the minimum required versions for the main
-         dependencies listed above. In that case, download, inspect and execute
-         the Kitware archive script to add the Kitware APT repository to your
-         sources list.
-         A detailed explanation of ``kitware-archive.sh`` can be found here
-         `kitware third-party apt repository <https://apt.kitware.com/>`_:
-
-         .. code-block:: bash
-
-            wget https://apt.kitware.com/kitware-archive.sh
-            sudo bash kitware-archive.sh
-
       #. Use ``apt`` to install the required dependencies:
 
          .. code-block:: bash