fix: downgrade mongodb-memory-server to 6.x to support node 8.x

Author: vladholubiev

package.json

@@ -43,7 +43,7 @@
   },
   "dependencies": {
     "debug": "4.3.2",
-    "mongodb-memory-server": "7.2.0",
+    "mongodb-memory-server": "6.10.0",
     "uuid": "8.3.2"
   },
   "devDependencies": {