[Maps] Fix text readability on map scale, attribution, and coordinate…

… controls (elastic#189639)

## Summary

Fixes elastic#128783

This PR fixes the visualization of the texts in Maps scale bar,
attribution, and coordinate controls with a dark background.

## Context

While checking on the scale bar component of the map I saw this SCSS
mixin that seemed to be broken.

Originally I thought on fixing the shadow but the readability of the
text is not great for dark background (see
7f2a698)

<details><summary>Adding a shadow</summary>

| before | after |
| :--:| :--:| 

|![2024-07-31_16-17](https://github.com/user-attachments/assets/23239b0d-826e-482e-ac67-fe7faa50d696)
|
![2024-07-31_16-18](https://github.com/user-attachments/assets/dffe0873-86dd-46b3-877e-ff4f5ef92c32)
|
</details>

So I decided to propose to fix this by adding a background as is usual
in other platforms.


![image](https://github.com/user-attachments/assets/7a7d0d11-1cf6-4961-b872-a5247c930363)


![image](https://github.com/user-attachments/assets/2c3d893b-f380-4724-9ad5-93adb6ec9fb6)


![image](https://github.com/user-attachments/assets/1a9becbc-40b1-4631-8360-f2a1bfafc9c2)

x-pack/plugins/maps/public/connected_components/mb_map/scale_control/_index.scss

@@ -5,9 +5,10 @@
   bottom: $euiSizeM;
   pointer-events: none;
   color: $euiTextColor;
-  border-left: 2px solid $euiTextColor;
-  border-bottom: 2px solid $euiTextColor;
+  border-left: 2px solid rgba($euiTextColor, .6);
+  border-bottom: 2px solid rgba($euiTextColor, .6);
   text-align: right;
+  @include mapOverlayIsTextOnly;
 }
 
 .mapScaleControlFullScreen {

x-pack/plugins/maps/public/connected_components/right_side_controls/_mixins.scss

@@ -1,8 +1,6 @@
 @mixin mapOverlayIsTextOnly {
-  text-shadow:
-    0 0 2px $euiColorEmptyShade,
-    0 0 1px $euiColorEmptyShade,
-    0 0 1px $euiColorEmptyShade,
-    0 0 1px $euiColorEmptyShade,
-    0 0 1px $euiColorEmptyShade      0 0 1px $euiColorEmptyShade      0 0 1px $euiColorEmptyShade;
+  background-color: $euiPageBackgroundColor;
+
+  padding-left: $euiSizeXS;
+  padding-right: $euiSizeXS;
 }

x-pack/plugins/maps/public/connected_components/right_side_controls/attribution_control/_attribution_control.scss

@@ -2,7 +2,6 @@
   @include mapOverlayIsTextOnly;
   @include euiTextBreakWord;
   pointer-events: all;
-  padding-left: $euiSizeM;
 }
 
 .mapAttributionControl__fullScreen {