Prevent vuln table header from cutting off dropdown

Instead of z-index racing, make a new stacking context for the
table.

The checkboxes have z-index: 1 (from bootstrap), which caused
them to float above the header (so that _does_ need z-index),
but everything outside the table should be stacked above it.

Changelog: fixed
EE: true

ee/app/assets/javascripts/security_dashboard/components/shared/selection_summary.vue

@@ -109,7 +109,7 @@ export default {
 <template>
   <gl-collapse
     :visible="visible"
-    class="selection-summary gl-z-index-3!"
+    class="selection-summary"
     data-testid="selection-summary-collapse"
   >
     <div class="card" :class="{ 'with-error': Boolean(updateErrorText) }">

ee/app/assets/stylesheets/components/vulnerability_list.scss

@@ -15,6 +15,8 @@ $selection-summary-with-error-height: 118px;
 }
 
 .vulnerability-list {
+  isolation: isolate;
+
   @media (min-width: $breakpoint-sm) {
     .checkbox {
       @include gl-pl-4;