Refactor user_id column type from unsignedInteger to unsignedBigInteger

[milad-abdi]

In this pull request, the user_id column in the migration file has been updated from unsignedInteger to unsignedBigInteger. This change ensures better compatibility, especially when the id column in the users table is of type unsignedBigInteger.

The reason for this change is to handle larger integer values and to prevent potential issues when referencing the users table with BIGINT primary keys. It also aligns with Laravel's default use of unsignedBigInteger for the id column in the users table, ensuring proper foreign key relationships.

This update is backward-compatible as the column remains nullable, and no additional changes to the database structure are required beyond this modification.