Fix focus-visibility for vulnerability actions

When navigating through the security dashboard using the keyboard,
the action buttons for a given vulnerability stay hidden,
even when they have focus.

This commit changes the responsible CSS-selector from using `:focus`
to `:focus-within` so the row of buttons is set to be visible whenever
an element inside has focus.

More info:
https://developer.mozilla.org/en-US/docs/Web/CSS/:focus-within

ee/app/assets/javascripts/security_dashboard/components/security_dashboard_table_row.vue

@@ -143,7 +143,7 @@ export default {
   }
 
   .vulnerabilities-row:hover .action-buttons,
-  .vulnerabilities-row:focus .action-buttons {
+  .vulnerabilities-row:focus-within .action-buttons {
     opacity: 1;
   }
 }

ee/changelogs/unreleased/sec-dashboard-vulnerability-table-row-action-buttons-focus-fix.yml

+---
+title: Fix focus-visibility of vulnerability-actions within security
+  dashboard 
+merge_request: 15115
+author:
+type: fixed