Update pagination focus styles

- Drops the hover-focus mixin for standard :hover
- Adds explicit :focus styles to match button, input, etc with a box-shadow
- Adjusts z-index values to ensure the layers go initial, hover, active, focus

Fixes #24838.

scss/_variables.scss

@@ -627,6 +627,8 @@ $pagination-bg:                     $white !default;
 $pagination-border-width:           $border-width !default;
 $pagination-border-color:           $gray-300 !default;
 
+$pagination-focus-box-shadow:       $input-btn-focus-box-shadow !default;
+
 $pagination-hover-color:            $link-hover-color !default;
 $pagination-hover-bg:               $gray-200 !default;
 $pagination-hover-border-color:     $gray-300 !default;