Fix some self deprecation notices

src/Event/Subscriber/DownloadImagesSubscriber.php

@@ -25,7 +25,7 @@ class DownloadImagesSubscriber implements EventSubscriberInterface
         $this->logger = $logger;
     }
 
-    public static function getSubscribedEvents()
+    public static function getSubscribedEvents(): array
     {
         return [
             EntrySavedEvent::NAME => 'onEntrySaved',

src/Event/Subscriber/GenerateCustomCSSSubscriber.php

@@ -18,7 +18,7 @@ class GenerateCustomCSSSubscriber implements EventSubscriberInterface
         $this->compiler = $compiler;
     }
 
-    public static function getSubscribedEvents()
+    public static function getSubscribedEvents(): array
     {
         return [
             ConfigUpdatedEvent::NAME => 'onConfigUpdated',

src/Event/Subscriber/SchemaAdapterSubscriber.php

@@ -15,7 +15,7 @@ class SchemaAdapterSubscriber implements EventSubscriber
         $this->databaseTablePrefix = $databaseTablePrefix;
     }
 
-    public function getSubscribedEvents()
+    public function getSubscribedEvents(): array
     {
         return ['postGenerateSchema'];
     }

src/Event/Subscriber/TablePrefixSubscriber.php

@@ -25,7 +25,7 @@ class TablePrefixSubscriber implements EventSubscriber
         $this->tablePrefix = (string) $tablePrefix;
     }
 
-    public function getSubscribedEvents()
+    public function getSubscribedEvents(): array
     {
         return ['loadClassMetadata'];
     }