Make id column as AUTO INCREMENT on plugin activation

Add the autoincrement keyword after `id` int NOT NULL:

public static function activate() {
    global $wpdb;

    $sql = "CREATE TABLE `{$wpdb->base_prefix}employee` (
    `id` int NOT NULL autoincrement,
    `name` varchar(255) NOT NULL,
    `dob` date NOT NULL,
    `gender` int NOT NULL,
    `email` varchar(255) NOT NULL,
    `joining_date` date NOT NULL,
    `address` varchar(255) NOT NULL,
    `salary_package` int NOT NULL,
    `marital_status` int NOT NULL,
    `department` varchar(255) NOT NULL,
    `designation` varchar(255) NOT NULL,
    PRIMARY KEY (id)
  ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci AUTO_INCREMENT=1;
  COMMIT;";

    require_once(ABSPATH . 'wp-admin/includes/upgrade.php');
    dbDelta($sql);
}