C# Pong: How to increase ball speed over time?

So im currently working on a simple Pong game in Unity. I've got everything working out fine so far but I can't seem to figure out how to get the ball speed to increase over time. Here's my script which is attached to the ball:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class ballMovement : MonoBehaviour
 {
     public float speed = 10;
 
     void Start()
     {
         // Initial Velocity
         GetComponent<Rigidbody2D>().velocity = Vector2.right * speed;
     }
 
     float hitFactor(Vector2 ballPos, Vector2 racketPos,
                 float racketHeight)
     {
         // ascii art:
         // ||  1 <- at the top of the racket
         // ||
         // ||  0 <- at the middle of the racket
         // ||
         // || -1 <- at the bottom of the racket
         return (ballPos.y - racketPos.y) / racketHeight;
     }
 
     void OnCollisionEnter2D(Collision2D col)
     {
         if (col.gameObject.name == "RacketLeft")
         {
             // Calculate hit Factor
             float y = hitFactor(transform.position,
                                 col.transform.position,
                                 col.collider.bounds.size.y);
 
             // Calculate direction, make length=1 via .normalized
             Vector2 dir = new Vector2(1, y).normalized;
 
             // Set Velocity with dir * speed
             GetComponent<Rigidbody2D>().velocity = dir * speed;
         }
 
         // Hit the right Racket?
         if (col.gameObject.name == "RacketRight")
         {
             // Calculate hit Factor
             float y = hitFactor(transform.position,
                                 col.transform.position,
                                 col.collider.bounds.size.y);
 
             // Calculate direction, make length=1 via .normalized
             Vector2 dir = new Vector2(-1, y).normalized;
 
             // Set Velocity with dir * speed
             GetComponent<Rigidbody2D>().velocity = dir * speed;
         }
     }
 
 }