Stop claiming to support Debian distros

We haven't had a working job for stretch or jessie in years, attempts
to get things fixed have been dropped, set let's be honest and drop
those from our support list.

Change-Id: Ia6152be79f8044f7ff039ec0911ad4938d6271f4
diff --git a/stack.sh b/stack.sh
index fa27743..709b97b 100755
--- a/stack.sh
+++ b/stack.sh
@@ -221,7 +221,7 @@
 
 # Warn users who aren't on an explicitly supported distro, but allow them to
 # override check and attempt installation with ``FORCE=yes ./stack``
-if [[ ! ${DISTRO} =~ (bionic|focal|stretch|jessie|f30|f31|opensuse-15.0|opensuse-15.1|opensuse-tumbleweed|rhel8) ]]; then
+if [[ ! ${DISTRO} =~ (bionic|focal|f30|f31|opensuse-15.0|opensuse-15.1|opensuse-tumbleweed|rhel8) ]]; then
     echo "WARNING: this script has not been tested on $DISTRO"
     if [[ "$FORCE" != "yes" ]]; then
         die $LINENO "If you wish to run this script anyway run with FORCE=yes"