[PHP-Symfony] Fix Symfony warning on OpenAPIServerBundle (#15960)

This fixes this Symfony warning:

    User Deprecated: Method "Symfony\Component\HttpKernel\Bundle\Bundle::build()"
    might add "void" as a native return type declaration in the future. Do the
    same in child class "OpenAPI\Server\OpenAPIServerBundle" now to avoid errors
    or add an explicit @return annotation to suppress this message

This fixes #15959

modules/openapi-generator/src/main/resources/php-symfony/Bundle.mustache

@@ -33,7 +33,7 @@ use {{invokerPackage}}\DependencyInjection\Compiler\{{bundleName}}ApiPass;
  */
 class {{bundleClassName}} extends Bundle
 {
-    public function build(ContainerBuilder $container)
+    public function build(ContainerBuilder $container): void
     {
         $container->addCompilerPass(new {{bundleName}}ApiPass());
     }

samples/server/petstore/php-symfony/SymfonyBundle-php/OpenAPIServerBundle.php

@@ -43,7 +43,7 @@
  */
 class OpenAPIServerBundle extends Bundle
 {
-    public function build(ContainerBuilder $container)
+    public function build(ContainerBuilder $container): void
     {
         $container->addCompilerPass(new OpenAPIServerApiPass());
     }