ソースコード

using UnityEngine;

[DisallowMultipleComponent]
public sealed class Example : MonoBehaviour
{
    [SerializeField] private float m_maxSpeed;

    private void FixedUpdate()
    {
        var rigidbody2D = GetComponent<Rigidbody2D>();

        rigidbody2D.velocity = Vector3.ClampMagnitude
        (
            vector: rigidbody2D.velocity,
            maxLength: m_maxSpeed
        );
    }
}