Discover table disappears when dragging fields from the field list (#209714)

## Summary

Closes #209442 

After dropping [color utility
functions](https://eui.elastic.co/#/theming/colors/utilities)
accidentally drag and drop zone background became solid, hiding table
content.

![image](https://github.com/user-attachments/assets/5c5b9686-ee11-4407-909f-56c53e4a0cd1)

EUI team agreed to revert `transparentize()` in this particular case
(until migration from SCSS to Emotions, when a new proper token should
already exist). Current color tokens though demanded a bit different
opacity values than previously. I was trying to achieve the most similar
effect to the `8.17` version.

I also adjusted border as it was a bit too light compared to `8.17`.

**8.17 border:**
<img width="176" alt="Screenshot 2025-02-05 at 10 43 35"
src="https://github.com/user-attachments/assets/bf71747a-63a5-4630-8a70-674a9576dbe0"
/>


**`main` border**
<img width="181" alt="Screenshot 2025-02-05 at 10 34 39"
src="https://github.com/user-attachments/assets/c0756f20-ae74-4c72-914c-3b9fde96faa0"
/>

**My choice from existing tokens (there's nothing in between `base` and
`strong` success for borders):**
<img width="174" alt="Screenshot 2025-02-05 at 10 42 24"
src="https://github.com/user-attachments/assets/584f5577-4842-4b44-a395-3b0bcbaf4c82"
/>


Below you can find gifs with both versions in action. I'm open to any
other color-opacity pairs suggestions if those don't mirror the previous
state enough.

**8.17**
![8 17 drag and
drop](https://github.com/user-attachments/assets/a95db5da-1af1-4373-a58d-6ecda33f9387)

**This PR**
![PR drag and
drop](https://github.com/user-attachments/assets/cf6c1a1e-8f8d-4462-8984-88837289ccd1)


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Describe the risk, its severity, and mitigation for each identified
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---------

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

src/platform/packages/shared/kbn-dom-drag-drop/src/sass/drag_drop_mixins.scss

@@ -39,18 +39,20 @@ $domDragDropZLevel3: 3;
 }
 
 // Style for drop area when there's an item being dragged
+// Until SCSS -> emotions migration we should continue using "transparentize()" here
 @mixin mixinDomDroppableActive($borderWidth: $euiBorderWidthThin) {
-  background-color: $euiColorBackgroundBaseSuccess !important;
+  background-color: transparentize($euiColorBackgroundLightSuccess, .7) !important;
   &:before {
-    border-color: $euiColorBorderBaseSuccess !important;
+    border-color: $euiColorBorderStrongSuccess !important;
   }
 }
 
 // Style for drop area while hovering with item
+// Until SCSS -> emotions migration we should continue using "transparentize()" here
 @mixin mixinDomDroppableHover($borderWidth: $euiBorderWidthThin) {
-  background-color: $euiColorBackgroundLightSuccess !important;
+  background-color: transparentize($euiColorBackgroundFilledSuccess, .75) !important;
   &:before {
-    border-color: $euiColorBorderBaseSuccess !important;
+    border-color: $euiColorBorderStrongSuccess !important;
   }
 }