Upgrade to mongoose v8

[ypicard]

Here is my attempt to add support to Mongoose v8 to this package. The changes here are solely related to the mongoose migration guides:

• v5 to v6:
  • mongoose.connect() returns a Promise
• v6 to v7:
  • Dropped callback support
• v7 to v8:
  • Nothing.

No changes in behavior should be introduced in this PR.

I will run this version on my application for the next couple days and inform you if anything needs additional changes.

[Giggiux]

I've been using @ypicard's repo locally and seems to work quite well without issues! Would it be possible to merge this PR so that a new version of mongoose-aggregate-paginate-v2 is available in NPM too?

[aravindnc]

Thank you @ypicard @Giggiux