[Console] Command::execute() should always return int - deprecate returning null

- added deprecation message for non-int return value in Command::execute()
- fixed all core commands to return proper int values
- added proper return type-hint to Command::execute() method in all core Commands

Author: jschaedl

Command/DebugCommand.php

@@ -111,12 +111,16 @@ protected function execute(InputInterface $input, OutputInterface $output)
 
         switch ($input->getOption('format')) {
             case 'text':
-                return $name ? $this->displayPathsText($io, $name) : $this->displayGeneralText($io, $filter);
+                $name ? $this->displayPathsText($io, $name) : $this->displayGeneralText($io, $filter);
+                break;
             case 'json':
-                return $name ? $this->displayPathsJson($io, $name) : $this->displayGeneralJson($io, $filter);
+                $name ? $this->displayPathsJson($io, $name) : $this->displayGeneralJson($io, $filter);
+                break;
             default:
                 throw new InvalidArgumentException(sprintf('The format "%s" is not supported.', $input->getOption('format')));
         }
+
+        return 0;
     }
 
     private function displayPathsText(SymfonyStyle $io, string $name)