Merge branch '5.4' into 6.2

* 5.4:
  Remove usage of constant for better consistency across the codebase
  [HttpClient] Ensure HttplugClient ignores invalid HTTP headers

Author: fabpot

Command/CompleteCommand.php

@@ -173,10 +173,10 @@ protected function execute(InputInterface $input, OutputInterface $output): int
                 throw $e;
             }
 
-            return self::FAILURE;
+            return 2;
         }
 
-        return self::SUCCESS;
+        return 0;
     }
 
     private function createCompletionInput(InputInterface $input): CompletionInput

Command/DumpCompletionCommand.php

@@ -96,7 +96,7 @@ protected function execute(InputInterface $input, OutputInterface $output): int
         if ($input->getOption('debug')) {
             $this->tailDebugLog($commandName, $output);
 
-            return self::SUCCESS;
+            return 0;
         }
 
         $shell = $input->getArgument('shell') ?? self::guessShell();
@@ -113,12 +113,12 @@ protected function execute(InputInterface $input, OutputInterface $output): int
                 $output->writeln(sprintf('<error>Shell not detected, Symfony shell completion only supports "%s").</>', implode('", "', $supportedShells)));
             }
 
-            return self::INVALID;
+            return 2;
         }
 
         $output->write(str_replace(['{{ COMMAND_NAME }}', '{{ VERSION }}'], [$commandName, CompleteCommand::COMPLETION_API_VERSION], file_get_contents($completionFile)));
 
-        return self::SUCCESS;
+        return 0;
     }
 
     private static function guessShell(): string