add postinst doc

fix php 7.3 compatibility issues of typed properties

api/src/handlers/RequestBase.php

@@ -2,8 +2,8 @@
 require_once 'src/Database.php';
 
 abstract class RequestBase {
-    protected mysqli $conn;
-    private array $actions = array();
+    protected $conn;
+    private $actions = array();
 
     /**
      * adds a new action handler to the current api file

api/src/handlers/Video.php

@@ -7,7 +7,7 @@ require_once 'RequestBase.php';
  * backend for all interactions with videoloads and receiving of video infos
  */
 class Video extends RequestBase {
-    private string $videopath;
+    private $videopath;
 
     public function __construct() {
         $settings = new SSettings();