$delta is passed as string and not integer

This is a follow-up for #2973615: Add Section delta as extra data on filtered block definitions. $delta is passed as a string but should be passed as a integer.

i.e.

+++ b/core/modules/layout_builder/src/Controller/ChooseBlockController.php
@@ -63,8 +63,13 @@ public function build(SectionStorageInterface $section_storage, $delta, $region)
+    // Currently delta is passed as a string, so we'll explicitily convert it
+    // to a integer.
+    $delta = (int) $delta;