if (rb.velocity.y < 0) {
    rb.velocity += Vector3.up * Physics.gravity.y * (fallMultiplier - 1) * Time.deltaTime;
} else if (rb.velocity.y > 0 && !Input.GetButton("Jump")) {
    rb.velocity += Vector3.up * Physics.gravity.y * (lowJumpMultiplier - 1) * Time.deltaTime;
}