docs/devel: update references to centos to non-versioned container

>From the website:

"After May 31, 2024, CentOS Stream 8 will be archived and no further
updates will be provided."

We have updated a few bits but there are still references that need
fixing. Rather than bump I've replaced them with references to the
Debian image so we don't have to bump at the next update.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240603175328.3823123-3-alex.bennee@linaro.org>

docs/devel/testing.rst

@@ -387,9 +387,9 @@ make target):
 
 .. code::
 
-  make docker-test-build@centos8
+  make docker-test-build@debian
 
-This will create a container instance using the ``centos8`` image (the image
+This will create a container instance using the ``debian`` image (the image
 is downloaded and initialized automatically), in which the ``test-build`` job
 is executed.
 
@@ -410,8 +410,8 @@ locally by using the ``NOCACHE`` build option:
 Images
 ~~~~~~
 
-Along with many other images, the ``centos8`` image is defined in a Dockerfile
-in ``tests/docker/dockerfiles/``, called ``centos8.docker``. ``make docker-help``
+Along with many other images, the ``debian`` image is defined in a Dockerfile
+in ``tests/docker/dockerfiles/``, called ``debian.docker``. ``make docker-help``
 command will list all the available images.
 
 A ``.pre`` script can be added beside the ``.docker`` file, which will be