[Rector] Applied all Symfony 5.x rectors to the production codebase

Applied rules:
 * AddReturnTypeDeclarationRector

src/bundle/DependencyInjection/Configuration.php

@@ -12,7 +12,7 @@
 
 class Configuration extends SiteAccessConfiguration
 {
-    public function getConfigTreeBuilder()
+    public function getConfigTreeBuilder(): TreeBuilder
     {
         $treeBuilder = new TreeBuilder(IbexaDesignEngineExtension::EXTENSION_NAME);
 

src/bundle/DependencyInjection/IbexaDesignEngineExtension.php

@@ -8,6 +8,7 @@
 namespace Ibexa\Bundle\DesignEngine\DependencyInjection;
 
 use Ibexa\Bundle\Core\DependencyInjection\Configuration\SiteAccessAware\ConfigurationProcessor;
+use Symfony\Component\Config\Definition\ConfigurationInterface;
 use Symfony\Component\Config\FileLocator;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\DependencyInjection\Loader;
@@ -17,12 +18,12 @@ class IbexaDesignEngineExtension extends Extension
 {
     public const EXTENSION_NAME = 'ibexa_design_engine';
 
-    public function getAlias()
+    public function getAlias(): string
     {
         return self::EXTENSION_NAME;
     }
 
-    public function getConfiguration(array $config, ContainerBuilder $container)
+    public function getConfiguration(array $config, ContainerBuilder $container): ?ConfigurationInterface
     {
         return new Configuration();
     }

src/bundle/IbexaDesignEngineBundle.php

@@ -14,6 +14,7 @@
 use Ibexa\Bundle\DesignEngine\DependencyInjection\IbexaDesignEngineExtension;
 use Symfony\Component\DependencyInjection\Compiler\PassConfig;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
+use Symfony\Component\DependencyInjection\Extension\ExtensionInterface;
 use Symfony\Component\HttpKernel\Bundle\Bundle;
 
 class IbexaDesignEngineBundle extends Bundle
@@ -32,7 +33,7 @@ public function build(ContainerBuilder $container)
         $container->addCompilerPass(new AssetPathResolutionPass(), PassConfig::TYPE_OPTIMIZE);
     }
 
-    public function getContainerExtension()
+    public function getContainerExtension(): ?ExtensionInterface
     {
         if (!isset($this->extension)) {
             $this->extension = new IbexaDesignEngineExtension();