Do not support fieldName 'length' on Mongo 4.X

[Moumouls]

New Issue Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.
• I can reproduce the issue with the latest versions of Parse Server and the Parse JS SDK.

Issue Description

Issue name

Steps to reproduce

Run Parse SDK JS integration test with a mongo version 4.X

Actual Outcome

Cannot save object with a field "length"

Expected Outcome

Can save the object

Environment

Server

• Parse Server version: master
• Operating system: macOS
• Local or remote host (AWS, Azure, Google Cloud, Heroku, Digital Ocean, etc): local

Database

• System (MongoDB or Postgres): Mongo
• Database version: 4.X
• Local or remote host (MongoDB Atlas, mLab, AWS, Azure, Google Cloud, etc): local

Client

• Parse JS SDK version: master

Logs

Failures:
1) Parse LocalDatastore Parse Query Pinning (Default) Default supports objects with length
  Message:
    Error: Invalid field name: length.
  Stack:
    error properties: Object({ code: 105 })
    Error: Invalid field name: length.
        at handleError (/Users/antoinecormouls/Desktop/OpenSource/Parse-SDK-JS/lib/node/RESTController.js:422:17)
        at <Jasmine>
        at processTicksAndRejections (internal/process/task_queues.js:97:5)

2) Parse LocalDatastore Parse Query Pinning (React-Native) React-Native supports objects with length
  Message:
    Error: Invalid field name: length.
  Stack:
    error properties: Object({ code: 105 })
    Error: Invalid field name: length.
        at handleError (/Users/antoinecormouls/Desktop/OpenSource/Parse-SDK-JS/lib/node/RESTController.js:422:17)
        at <Jasmine>
        at processTicksAndRejections (internal/process/task_queues.js:97:5)

3) Parse Query supports objects with length
  Message:
    Unhandled promise rejection: ParseError: 105 Invalid field name: length.
  Stack:
    error properties: Object({ code: 105 })
    Error: Invalid field name: length.
        at handleError (/Users/antoinecormouls/Desktop/OpenSource/Parse-SDK-JS/lib/node/RESTController.js:422:17)
        at <Jasmine>
        at processTicksAndRejections (internal/process/task_queues.js:97:5)

[dplewis]

This is expected behavior.

parse-community/parse-server#7053

I can open a PR to fix the tests since the server hasn’t been released yet.