Fix response model for DELETE /group endpoint

[JenySadadia]

Related to #321

API endpoint DELETE /group/<group-id> is implemented for deleting user groups matching provided group ID.

The handler delete_group has a pagination response model PageModel even if the endpoint is not returning anything. This adjustment was required to use DB method db.find_by_attributes method in the handler to get the required information before deleting the group. As the DB method is returning paginated response, I needed to declare response_model=PageModel in the handler to use it as per fastapi-pagination package requirements. Please see the comment for more details on the issue.

This can be fixed by using pagination_ctx from fastapi-pagination package that was introduced in version 0.12.0 as mentioned in uriyyo/fastapi-pagination#590. However, it requires fastapi>=0.80.0 version to use it.

Below is the error log when I tried to install fastapi-pagination 0.12.0:

ERROR: Cannot install -r docker/api/requirements.txt (line 4) and fastapi[all]==0.68.1 because these package versions have conflicting dependencies.

The conflict is caused by:
    fastapi[all] 0.68.1 depends on fastapi 0.68.1 (from https://files.pythonhosted.org/packages/df/44/ee1976b03404318590bbe4b0ef27007ce2c42b15757aa0c72bc99a4ebae7/fastapi-0.68.1-py3-none-any.whl#sha256=94d2820906c36b9b8303796fb7271337ec89c74223229e3cfcf056b5a7d59e23 (from https://pypi.org/simple/fastapi/) (requires-python:>=3.6))
    fastapi-pagination 0.12.0 depends on fastapi>=0.80.0

Checklist to complete the issue:

• Upgrade fastapi
• Upgrade fastapi-pagination
• Fix delete_group handler
• Test on staging