Replace use of 'array_merge' with the spread operator and 'array_push' in various places.

This isn't supported for arrays with string keys until PHP 8.1.

https://wiki.php.net/rfc/spread_operator_for_array

plugins/cache_starred_images/init.php

@@ -84,10 +84,10 @@ class Cache_Starred_Images extends Plugin {
 
 		Debug::log("expiring {$this->cache->get_dir()} and {$this->cache_status->get_dir()}...");
 
-		$files = array_merge(
-			glob($this->cache->get_dir() . "/*-*"),
-			glob($this->cache_status->get_dir() . "/*.status")
-		);
+		$files = [
+			...(glob($this->cache->get_dir() . "/*-*") ?: []),
+			...(glob($this->cache_status->get_dir() . "/*.status") ?: []),
+		];
 
 		asort($files);