postgresql@18: add stop_timeout 120

When PostgreSQL is killed abruptly (SIGKILL), it leaves behind postmaster.pid
and lock files in the data directory, preventing a subsequent start until those
files are manually removed. A 120-second graceful stop timeout allows PostgreSQL
to shut down cleanly before launchd escalates to a forced kill.

Author: IngmarStein

Formula/p/postgresql@18.rb

@@ -154,6 +154,7 @@ def caveats
     keep_alive true
     log_path f.postgresql_log_path
     error_log_path f.postgresql_log_path
+    stop_timeout 120
     working_dir HOMEBREW_PREFIX
   end