Upgrade mongoose to v6.x

[sherryhli]

Context:

• M0 clusters in Atlas may eventually be forced to upgrade to MongoDB v5.x (currently on v4.4.x)
• Currently starter code uses Mongoose v5.x which uses MongoDB Node.js driver v3.x under the hood. v3.x drivers are not compatible with MongoDB v5.x
• The solution is to upgrade to Mongoose v6.x, which uses MongoDB Node.js driver v4.x

Acceptance Criteria:

• Carefully follow the Mongoose v6 migration guide to upgrade our version of Mongoose, ensure that all breaking changes are handled
• Ensure that UserService and EntityService functions correctly when using the MongoDB implementation

Dev Notes:

• MongoDB Node.js driver compatibility: https://docs.mongodb.com/drivers/node/current/compatibility/
• Files to pay attention to:
  • backend/typescript/models/index.ts
  • backend/typescript/models/user.mgmodel.ts
  • backend/typescript/models/entity.mgmodel.ts
  • backend/typescript/services/implementations/EntityServiceMg.ts
  • backend/typescript/services/implementations/userService.ts

[gregmaxin]

• Upgraded Mongoose
• poolSize replaced with minPoolSize and maxPoolSize
• removed uses of useNewUrlParser, useUnifiedTopology, useFindAndModify, and useCreateIndex as they are no longer supported options.
• Changd mongoose connection to use asPromise()
• Checked for hardcoded errors - foudn none
• No sanitizeFilter Found
• No markModified Found