upgrade-zulip-stage-2: Set default PostgreSQL version for Debian 11.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

docs/development/setup-advanced.md

@@ -13,7 +13,7 @@ If you'd like to install a Zulip development environment on a computer
 that's running one of:
 
 - Ubuntu 20.04 Focal
-- Debian 10 Buster, 11 Bullseye (beta)
+- Debian 10 Buster, 11 Bullseye
 - CentOS 7 (beta)
 - Fedora 33 and 34 (beta)
 - RHEL 7 (beta)

scripts/lib/upgrade-zulip-stage-2

@@ -166,6 +166,7 @@ if os.path.exists("/etc/init.d/postgresql"):
     if not postgresql_version:
         default_postgresql_version = {
             ("debian", "10"): "11",
+            ("debian", "11"): "13",
             ("ubuntu", "20.04"): "12",
             ("centos", "7"): "11",
         }