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

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>