tests/dockerfiles: unify debian-toolchain references

We use the debian release number elsewhere so fix it for consistency
along with the broken comment.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230228190653.1602033-15-alex.bennee@linaro.org>

tests/docker/dockerfiles/debian-toolchain.docker

@@ -30,7 +30,7 @@ ADD build-toolchain.sh /root/build-toolchain.sh
 RUN cd /root && ./build-toolchain.sh
 
 # Throw away the extra toolchain build deps, the downloaded source,
-# and the build trees by restoring the original debian10 image,
+# and the build trees by restoring the original image,
 # then copying the built toolchain from stage 0.
-FROM docker.io/library/debian:bullseye-slim
+FROM docker.io/library/debian:11-slim
 COPY --from=0 /usr/local /usr/local