Forward port #11581 to 3.x

Use the md5-sum of group keys instead of the actual group names. This
helps mitigate key overflow issues in redis and memcached.

It does change the cache keys used, but those have never been promised
to be backwards compatible.

src/Cache/CacheEngine.php

@@ -252,7 +252,7 @@ public function key($key)
 
         $prefix = '';
         if ($this->_groupPrefix) {
-            $prefix = vsprintf($this->_groupPrefix, $this->groups());
+            $prefix = md5(implode('_', $this->groups()));
         }
 
         $key = preg_replace('/[\s]+/', '_', strtolower(trim(str_replace([DIRECTORY_SEPARATOR, '/', '.'], '_', (string)$key))));