refactor: PHP 8.5 language features via Rector

rector.php gets the PHP sets up to 8.5. Mostly constructor promotion with
readonly properties, typed class constants, ??=, arrow functions,
str_contains/str_starts_with, catch without unused variables, array_any
and #[\Override] on overridden properties; array_key_exists() keys are
cast to string (null keys are deprecated in PHP 8.5). Long promoted
constructor signatures are wrapped one parameter per line.

Checked locally: a crawl of ~1,900 frontend URLs over all sites and the
backchannel APIs (/api/hotel, /api/product, content feed manifest and
records of every type) render the same as before.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

packages/ep_theme/Classes/ViewHelpers/ContainerViewHelper.php

@@ -19,6 +19,7 @@ class ContainerViewHelper extends AbstractViewHelper
     /**
      * @var bool
      */
+    #[\Override]
     protected $escapeOutput = false;
 
     /**
@@ -45,9 +46,7 @@ class ContainerViewHelper extends AbstractViewHelper
     public function render()
     {
         $content = $this->renderChildren();
-        if ($content === null) {
-            $content = $this->arguments['content'];
-        }
+        $content ??= $this->arguments['content'];
         $classes = [];
         if (\in_array($this->arguments['data']['colPos'] ?? null, static::$columnsWithoutContainers, false)) {
             $classes[] = 'container';