remove unveil jquery plugin

### Circumstance

Remove unveil jquery plugin and instead use native browser lazy loading https://web.dev/browser-level-image-lazy-loading/.

In very special situations where it is not possible to use lazy parameter you should use observers as used on kudos for horizontal scroll https://gitcoin.co/kudos from the file v2/js/pages/carousel.js

### Why Is this Needed

Jquery unveil is an old plugin with low performance using old and expensive js to lazy load images, generating some issues on the site when the images don't load.

### Current Behavior

using unveil plugin

### Expected Behavior

using native lazy loading

### Definition of Done

- remove the library unveil
- switch from unveil to native lazy and observers when needed.