@uppy/dashboard: Fix low-contrast hover styles (#4347)

Fix low-contrast hover styles on Dashboard icons

packages/@uppy/core/src/_variables.scss

@@ -19,8 +19,8 @@ $beige: #edd4b9 !default;
 
 $gray-50: #fafafa !default;
 $gray-100: #f4f4f4 !default;
-$gray-100--highlighted: #f1f3f6 !default;
 $gray-200: #eaeaea !default;
+$gray-200--highlighted: #e9ecef !default;
 $gray-250: #dfdfdf !default;
 $gray-300: #cfcfcf !default;
 $gray-400: #bbb !default;

packages/@uppy/dashboard/src/style.scss

@@ -470,7 +470,7 @@
 }
 
 .uppy-DashboardTab-btn:hover {
-  background-color: $gray-100--highlighted;
+  background-color: $gray-200--highlighted;
 
   [data-uppy-theme="dark"] & {
     background-color: $gray-800;