scripts: Remove Xenial and Stretch support from installation scripts.

Note that we leave support for them in `setup-apt-repo` and puppet,
since we're still supporting systems using Xenial for non-appserver
puppet rules.

scripts/lib/check-upstart

@@ -1,11 +0,0 @@
-#!/usr/bin/env bash
-release=$(lsb_release -sc)
-
-if [ "$release" = "xenial" ] && [ -x /sbin/start ] && [ -x /sbin/stop ] && [ -x /sbin/restart ] && [ -x /sbin/status ] && [ -x /sbin/initctl ] && [ ! "$TRAVIS" ]; then
-    echo "You appear to be running Ubuntu Xenial, but with the upstart package installed."
-    echo "In Ubuntu Xenial, upstart has been replaced by systemd, and having upstart"
-    echo "installed will break the init scripts for Zulip dependencies like nginx."
-    echo "Please uninstall the \"upstart\" package (apt-get remove upstart) and rerun"
-    echo "this install script."
-    exit 1
-fi

scripts/lib/install

@@ -113,9 +113,7 @@ case "$os_id$os_version_id" in
 Unsupported OS release: $os_id $os_version_id
 
 Zulip in production is supported only on:
- - Debian 9 "stretch"
  - Debian 10 "buster"
- - Ubuntu 16.04 LTS "xenial"
  - Ubuntu 18.04 LTS "bionic"
 
 For more information, see:
@@ -167,9 +165,6 @@ case " $os_id $os_id_like " in
         ;;
 esac
 
-# Handle issues around upstart on Ubuntu Xenial
-"$ZULIP_PATH"/scripts/lib/check-upstart
-
 # Check early for missing SSL certificates
 if [ "$PUPPET_CLASSES" = "zulip::voyager" ] && [ -z "$USE_CERTBOT""$SELF_SIGNED_CERT" ] && { ! [ -e "/etc/ssl/private/zulip.key" ] || ! [ -e "/etc/ssl/certs/zulip.combined-chain.crt" ]; }; then
     set +x

scripts/lib/upgrade-zulip-stage-2

@@ -58,9 +58,6 @@ try:
 except (configparser.NoSectionError, configparser.NoOptionError):
     tornado_processes = 1
 
-# Handle issues around upstart on Ubuntu Xenial
-subprocess.check_call(["./scripts/lib/check-upstart"])
-
 if glob.glob("/usr/share/postgresql/*/extension/tsearch_extras.control"):
     # Remove legacy tsearch_extras package references
     subprocess.check_call([

tools/test-install/prepare-base

@@ -10,8 +10,6 @@ RELEASE="$1"
 ARCH=amd64  # TODO: maybe i686 too
 
 case "$RELEASE" in
-    xenial) extra_packages=(virtualenv postgresql-9.5)
-        ;;
     bionic) extra_packages=(virtualenv postgresql-10)
         ;;
     *)