Require PHP 7.4+ (#5720)

* Require PHP 7.4+
https://github.com/FreshRSS/FreshRSS/discussions/5474

* Update Docker oldest
Alpine 3.13 with PHP 7.4.26

* Add missing packets to Docker oldest

* Update to typed properties
https://php.net/migration74.new-features#migration74.new-features.core.typed-properties

* More types

app/Controllers/entryController.php

@@ -7,9 +7,8 @@ class FreshRSS_entry_Controller extends FreshRSS_ActionController {
 
 	/**
 	 * JavaScript request or not.
-	 * @var bool
 	 */
-	private $ajax = false;
+	private bool $ajax = false;
 
 	/**
 	 * This action is called before every other action in that class. It is