[Console Monaco] Improve color contrast of selection highlighting (el…

…astic#189060)

Closes elastic#186772

## Summary

This PR fixes the color contrast issue with the selection highlighting
in Console Monaco.

Before:
<img width="1378" alt="Screenshot 2024-07-24 at 12 49 09"
src="https://github.com/user-attachments/assets/a233127d-8fb7-4882-8d3f-e8dd9cf53210">

Now:
<img width="1378" alt="Screenshot 2024-07-24 at 11 59 08"
src="https://github.com/user-attachments/assets/d2bf79b6-b622-4a51-af18-8c7b4f6ddcd5">

Note: I changed the request highlighting color from grey to blue
(`euiColorPrimary`) since we are soon starting work on the Console
redesign project, in which the new request highlighting color is blue
(see screenshot from Figma project below). With the blue color, the
contrast seems to be much higher.

<details>
<summary>Screenshot from Figma project</summary>
<img width="966" alt="Screenshot 2024-07-24 at 12 53 55"
src="https://github.com/user-attachments/assets/c7afe72c-5d1c-43bd-a02e-e28547681441">


</details>

---------

Co-authored-by: kibanamachine <[email protected]>

src/plugins/console/public/styles/_app.scss

@@ -131,7 +131,7 @@
  * The highlighting for the selected requests in the monaco editor
  */
 .console__monaco_editor__selectedRequests {
-  background: transparentize($euiColorLightShade, .3);
+  background: transparentize($euiColorPrimary, .9);
 }
 /*
  * The z-index for the autocomplete suggestions popup