Change void return declarations commands to be integer returns

Calling these commands would throw a FatalErrorException: Compile Error:
A void function must not return a value

Since symfony commands require a proper exit code to be returned this
should be an integer return type

Author: konetchy

src/Command/CacheFlushAllCommand.php

@@ -33,7 +33,7 @@ public function configure(): void
         );
     }
 
-    public function execute(InputInterface $input, OutputInterface $output): void
+    public function execute(InputInterface $input, OutputInterface $output): int
     {
         $output->writeln('<info>Clearing cache information.</info>');
 

src/Command/CacheFlushCommand.php

@@ -42,7 +42,7 @@ public function configure(): void
     /**
      * @throws \RuntimeException
      */
-    public function execute(InputInterface $input, OutputInterface $output): void
+    public function execute(InputInterface $input, OutputInterface $output): int
     {
         $keys = @json_decode($input->getOption('keys'), true);