scripts: Fix check for services running when upgrading.

When upgrading from a pre-4.0 release, scripts/stop-server logic would
check whether supervisord configuration files were present to
determine what it needed to restart, but only considered paths to
those files that are introduced in Zulip 4.0.

Fixed #18493.

scripts/restart-server

@@ -17,6 +17,7 @@ from scripts.lib.zulip_tools import (
     get_config_file,
     get_tornado_ports,
     has_application_server,
+    has_process_fts_updates,
     overwrite_symlink,
 )
 
@@ -90,7 +91,7 @@ if has_application_server():
             worker_status.check_returncode()
         workers.extend(status_line.split()[0] for status_line in worker_status.stdout.splitlines())
 
-if os.path.exists("/etc/supervisor/conf.d/zulip/zulip_db.conf"):
+if has_process_fts_updates():
     workers.append("process-fts-updates")
 
 if action == "restart" and len(workers) > 0: