Remove support for building stretch

Must land only after cl/399073519 lands

BUG=b:1242140
TEST=kokoro

Change-Id: I1aa6f93a9a9a8f4c2ee17cf6514c50872ef629aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/containers/cros-container-guest-tools/+/3184844
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Reviewed-by: David Munro <davidmunro@google.com>
Commit-Queue: Fergus Dall <sidereal@google.com>
diff --git a/scripts/uprev.py b/scripts/uprev.py
index 2843452..e7268bc 100755
--- a/scripts/uprev.py
+++ b/scripts/uprev.py
@@ -52,7 +52,7 @@
             file_arch = arch
 
         for ctype in ['test', 'app_test']:
-            for release in ['stretch', 'buster', 'bullseye']:
+            for release in ['buster', 'bullseye']:
                 base_url = f'gs://cros-containers-staging/{milestone}' \
                     f'/images/debian/{release}/{arch}/{ctype}/{latest_container}/'