tests/docker: bump the xtensa base to debian:11-slim

Stretch is going out of support so things like security updates will
fail. As the toolchain itself is binary it hopefully won't mind the
underlying OS being updated.

Message-Id: <20230503091244.1450613-3-alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reported-by: Richard Henderson <richard.henderson@linaro.org>
(cherry picked from commit 3217b84)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

tests/docker/dockerfiles/debian-xtensa-cross.docker

@@ -5,7 +5,7 @@
 # using a prebuilt toolchains for Xtensa cores from:
 # https://github.com/foss-xtensa/toolchain/releases
 #
-FROM docker.io/library/debian:stretch-slim
+FROM docker.io/library/debian:11-slim
 
 RUN apt-get update && \
     DEBIAN_FRONTEND=noninteractive apt install -yy eatmydata && \