Change task status to assigned after assigning an available task

Description

When the superuser assigns an available task to some existing user the API call adds an assignee parameter in the task but does not change the status of the task, i.e. the status still remains available.

Debugging steps followed

• Api being used patch /task/:id
• First, we thought that it was a caching issue
• On further debugging, we concluded that the issue was because the backend was just changing the task assignee, not the task status

References for developer

• To know about the API have a look into its API contract over here: https://github.com/Real-Dev-Squad/website-api-contracts/blob/main/tasks/README.md#patch-tasksid

Screenshots for reference

Video for the issue

Expectations from the solution

• on assigning an available task to a user change its status to assigned
• add tests for checking if the status is updated in the DB.
• Please update the API contracts.
  - [ ] Similarly, when the user is removed from the assignee the task status should go back to available (mentioned the reason in the comments Update task status to assigned on successful assignment #651 (comment))