fix: expire_allocation api endpoint by fetching allocation object (backport #1576)

[mergify]

Add the code to fetch allocation in expire_allocation endpoint so that after code not break.

Traceback of error which is fixed

Traceback (most recent call last):
  File "apps/frappe/frappe/app.py", line 110, in application
    response = frappe.api.handle(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "apps/frappe/frappe/api/__init__.py", line 49, in handle
    data = endpoint(**arguments)
           ^^^^^^^^^^^^^^^^^^^^^
  File "apps/frappe/frappe/api/v1.py", line 36, in handle_rpc_call
    return frappe.handler.handle()
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "apps/frappe/frappe/handler.py", line 49, in handle
    data = execute_cmd(cmd)
           ^^^^^^^^^^^^^^^^
  File "apps/frappe/frappe/handler.py", line 85, in execute_cmd
    return frappe.call(method, **frappe.form_dict)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "apps/frappe/frappe/__init__.py", line 1682, in call
    return fn(*args, **newargs)
           ^^^^^^^^^^^^^^^^^^^^
  File "apps/frappe/frappe/utils/typing_validations.py", line 31, in wrapper
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "apps/hrms/hrms/hr/doctype/leave_ledger_entry/leave_ledger_entry.py", line 193, in expire_allocation
    leaves = get_remaining_leaves(allocation)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "apps/hrms/hrms/hr/doctype/leave_ledger_entry/leave_ledger_entry.py", line 178, in get_remaining_leaves
    "employee": allocation.employee,
                ^^^^^^^^^^^^^^^^^^^
AttributeError: 'str' object has no attribute 'employee'
```<hr>This is an automatic backport of pull request #1576 done by [Mergify](https://mergify.com).

[frappe-pr-bot]

🎉 This PR is included in version 15.29.0 🎉

The release is available on GitHub release

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