[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/ServerDumpCommand.php

@@ -75,7 +75,7 @@ protected function configure()
         ;
     }
 
-    protected function execute(InputInterface $input, OutputInterface $output)
+    protected function execute(InputInterface $input, OutputInterface $output): int
     {
         $io = new SymfonyStyle($input, $output);
         $format = $input->getOption('format');