MDL-81825 theme_boost: Refactor .sr-only usages for BS5

- Add .visually-hidden to the Boostratp 5 bridge SCSS file
- Replace .sr-only occurrences with .visually-hidden
- Replace .dropzone-sr-only-focusable  with .dropzone-visually-hidden-focusable
  for consistency

admin/classes/table/plugin_management_table.php

@@ -322,7 +322,7 @@ abstract class plugin_management_table extends flexible_table implements dynamic
             ],
             'title' => $labelstr,
             'label' => $labelstr,
-            'labelclasses' => 'sr-only',
+            'labelclasses' => 'visually-hidden',
         ];
 
         return $OUTPUT->render_from_template('core_admin/setting_configtoggle', $params);