Use _id Instead of usn for Teams CRUD

[devesh711]

Currently, the Teams CRUD operations are using the usn (University Serial Number) field as the primary identifier. This approach can lead to confusion and potential issues with uniqueness and consistency.

Proposal:

• Refactor all Teams CRUD operations to use the _id field as the primary identifier in the database and API endpoints.
• Update frontend and backend logic to reference _id instead of usn for all relevant actions (create, read, update, delete).