fix(atomic): collapsed answer gradient colour (#4081)

https://coveord.atlassian.net/browse/KIT-3313

Right now, the linear-gradient is hardcoded to white rather than to the
background atomic colour. This will create issues when clients modify
their background colour to something other than white, like we doc on
the docs site when in dark mode.

<img width="1222" alt="Screenshot 2024-06-11 at 3 34 27 PM"
src="https://github.com/coveo/ui-kit/assets/39384459/967e0fdc-923d-4554-a290-a795c7b66b66">

It doesn't seem possible for clients to change the linear-gradient.
Probably because it requires targeting a pseudo-element of a part with a
certain class. So, in this PR, I use the atomic-background colour
instead.

packages/atomic/src/components/common/generated-answer/styles/generated-answer.pcss

@@ -105,7 +105,7 @@
   }
   &.answer-collapsed:before {
     @apply h-full w-full absolute top-0 left-0 content-[''];
-    background: linear-gradient(transparent 11.25rem, white);
+    background: linear-gradient(transparent 11.25rem, var(--atomic-background));
   }
 }