Update Debian CI environments

Remove configuration that assumed that "oldstable" referred to Debian 9,
as this hasn't been the case since the Debian 11 release in
mid-2021 and Debian 9 is fully EOL at this point anyway.

That fixes error in CI:
E: The repository 'http://http.debian.net/debian stretch-backports Release' does not have a Release file.

Closes: iputils#462
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Noah Meyerhans <noahm@debian.org>
[ pvorel: keep generic distro names ]
Signed-off-by: Petr Vorel <pvorel@suse.cz>

ci/debian.sh

@@ -2,13 +2,6 @@
 # Copyright (c) 2019-2021 Petr Vorel <petr.vorel@gmail.com>
 set -ex
 
-if [ "$DISTRO_VERSION" = "oldstable" ]; then
-	cat <<EOF | tee /etc/apt/sources.list.d/stretch-backports.list
-deb http://http.debian.net/debian stretch-backports main contrib non-free
-EOF
-	BACKPORT_REPO="stretch-backports"
-fi
-
 if [ "$DISTRO_VERSION" = "xenial" ]; then
 	cat <<EOF | tee /etc/apt/sources.list.d/xenial-backports.list
 deb http://archive.ubuntu.com/ubuntu xenial-backports main restricted universe multiverse
@@ -35,7 +28,3 @@ apt install -y --no-install-recommends \
 	meson \
 	pkg-config \
 	xsltproc
-
-if [ "$BACKPORT_REPO" ]; then
-	apt install -y --no-install-recommends -t $BACKPORT_REPO meson
-fi