[12.x] Fix type casting for environment variables in config files (#6637)

* Update app.php

* Update cache.php

* Update database.php

* Update logging.php

* Update mail.php

config/logging.php

@@ -54,7 +54,7 @@ return [
 
         'stack' => [
             'driver' => 'stack',
-            'channels' => explode(',', env('LOG_STACK', 'single')),
+            'channels' => explode(',', (string) env('LOG_STACK', 'single')),
             'ignore_exceptions' => false,
         ],