drop php7.3 support, make use of 7.4 features

ext/qr_code/info.php

@@ -4,13 +4,13 @@ class QRImageInfo extends ExtensionInfo
 {
     public const KEY = "qr_code";
 
-    public $key = self::KEY;
-    public $name = "QR Codes";
-    public $url = "http://seemslegit.com";
-    public $authors = ["Zach Hall"=>"zach@sosguy.net"];
-    public $license = self::LICENSE_GPLV2;
-    public $description = "Turns BBCode into HTML";
-    public $documentation =
+    public string $key = self::KEY;
+    public string $name = "QR Codes";
+    public string $url = "http://seemslegit.com";
+    public array $authors = ["Zach Hall"=>"zach@sosguy.net"];
+    public string $license = self::LICENSE_GPLV2;
+    public string $description = "Turns BBCode into HTML";
+    public ?string $documentation =
 "Shows a QR Code for downloading a post to cell phones.
 Based on Artanis's Link to Post Extension <artanis.00@gmail.com>
 Further modified by Shish to remove the 7MB local QR generator and replace it with a link to google chart APIs";

ext/qr_code/main.php

@@ -3,7 +3,7 @@
 class QRImage extends Extension
 {
     /** @var QRImageTheme */
-    protected $theme;
+    protected ?Themelet $theme;
 
     public function onDisplayingImage(DisplayingImageEvent $event)
     {