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

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 3217b84f3cd813a7daffc64b26543c313f3a042a)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>