Switch to DateTimeImmutable everywhere

tests/Monolog/Handler/FleepHookHandlerTest.php

@@ -62,7 +62,7 @@ class FleepHookHandlerTest extends TestCase
             'level' => Logger::DEBUG,
             'level_name' => Logger::getLevelName(Logger::DEBUG),
             'channel' => 'channel',
-            'datetime' => new \DateTime(),
+            'datetime' => new \DateTimeImmutable(),
             'extra' => array(),
         );