Unity C# light angle does not change

Rotation in Unity is always a Quaternion. This has more than 3 components but 4: x, y, z, w which all move in range -1 to 1.

So this will never be anywhere near the range of 90°.

If you want to limit the rotation it is easier to do so in Euler angle space (the ones you know). You could do so using e.g.

Vector3 directionToTarget = target.position - transform.position;
// Rotation that would be required to fully look at the target
Quaternion lookRotation = Quaternion.LookRotation(directionToTarget);
// Euler space representation of the quaternion
Vector3 euler = lookRotation.eulerAngles;
// manipulate and clamp according to your needs
euler.x = Mathf.Clamp(euler.x, 90 - MaxAngle, 90 + MaxAngle);
// apply
transform.localRotation = Quaternion.Euler(euler);

See

Note that in general eulerAngles not always behaves as expected and seemingly the same looking Quaternion can have quite different Euler representations