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

phpunit.xml.dist

@@ -4,6 +4,7 @@
     <php>
         <ini name="error_reporting" value="-1"/>
         <ini name="max_execution_time" value="-1"/>
+        <ini name="date.timezone" value="UTC"/>
         <ini name="intl.default_locale" value="en_US"/>
         <env name="KERNEL_CLASS" value="App\Kernel" force="true"/>
         <env name="SYMFONY_DEPRECATIONS_HELPER" value="weak"/>