Use PHP 7.4 featers and PHP 8 polyfills

data/translatable-strings.php

@@ -9,7 +9,7 @@
 // Exit if accessed directly
 defined('ABSPATH') || exit;
 
-return array(
+return [
 	'day'    => __('day', 'wp-ultimo'),
 	'days'   => __('days', 'wp-ultimo'),
 	'week'   => __('week', 'wp-ultimo'),
@@ -18,4 +18,4 @@ return array(
 	'months' => __('months', 'wp-ultimo'),
 	'year'   => __('year', 'wp-ultimo'),
 	'years'  => __('years', 'wp-ultimo'),
-);
+];