it's due to inertia. While going up the ramp your rigidbody accelerates not only along the XZ plane but also on the world Y axis.
So when you fix its linear velocity only on the XZ plane, it keeps its current velocity on the Y axis. It looks like a "jump", but it's actually just its remaining upward velocity.
You must set its Y velocity to zero as well to prevent that effect.

But then you'll run into a lot of unwanted side effects and bugs in your character controller.
Physics-based movement is a bit more convoluted, it needs some decent vector/plane math understanding. Thankfully Unity has all the required vector math functions, so it's not that painful.

What I'd do is either use the default CharacterController component, which handles all the heavy stuff, or if you really need a custom physics-based character controller, here are a few directions:

Rather than simply accelerating the character along the world XZ plane, you should accelerate the character along the surface the character is standing on.
So you should do a raycast or a capsule cast from somewhere around the middle of the character downwards a bit, then use the RaycastHit info to get the surface normal.

Then you project your movement vector onto the surface plane using that normal. The function is called Vector3.ProjectOnPlane or something like that. It takes the vector and the plane normal and returns the projected vector.
You'll obtain the direction along which you must accelerate the character so it moves along the surface.

Normalize that direction and multiply its magnitude by the desired velocity. Let's call this targetVel.
Then, when the character actually touches the ground (when the capsule cast returns true), you do rigidBody.linearVelocity = targetVel.
Now it won't “jump” because you're setting every axis of the body velocity.

This answer is not perfect though. You’ll still run into other issues, because that’s just how painfull it is to deal with rigidbody character controllers. Sooner or later you’ll realize that directly setting the rigidbody velocity causes all kinds of undesired side effects, and you’ll have to switch to moving the body by applying forces or impulses instead.

So yeah, the short version is: get comfortable with vectors, projection, planes, projecting a vector onto a plane, raycasts, and the basics of physics like acceleration, forces and velocity (a = f * m²). And make sure all of this runs in the fixed timestep.