prepare release 1.26.0 (#3554)

* bump version
* suppress deprecation
* composer update to fix twig security issue
* make sure to use the same timezone in all test runs

src/Configuration/ThemeConfiguration.php

@@ -29,6 +29,7 @@ final class ThemeConfiguration implements \ArrayAccess
     /**
      * @return mixed
      */
+    #[\ReturnTypeWillChange]
     public function offsetGet($offset)
     {
         return $this->systemConfiguration->find('theme.' . $offset);