installer: Sync the claimed supported distros with the check.

0f4b1076ad removed Ubuntu 16.04 "xenial" and Debian 9 "stretch" from
the printed list of supported operating systems, but left them in the
verification check that controls if that message is printed,
effectively continuing to support them.

Conversely, 439f0d3004 added Ubuntu 20.04 "focal" to the check, but
not to the printed list.

Synchronize to check and print the right supported distributions:
Ubuntu 18.04 "bionic", Ubuntu 20.04 "focal", and Debian 10 "buster".

scripts/lib/install

@@ -118,7 +118,7 @@ if [ -f /etc/os-release ]; then
 fi
 
 case "$os_id$os_version_id" in
-    ubuntu16.04|ubuntu18.04|debian9|debian10|ubuntu20.04) ;;
+    debian10|ubuntu18.04|ubuntu20.04) ;;
     *)
         set +x
         cat <<EOF
@@ -128,6 +128,7 @@ Unsupported OS release: $os_id $os_version_id
 Zulip in production is supported only on:
  - Debian 10 "buster"
  - Ubuntu 18.04 LTS "bionic"
+ - Ubuntu 20.04 LTS "focal"
 
 For more information, see:
   https://zulip.readthedocs.io/en/latest/production/requirements.html