MDL-68312 gradereport_grader: Use caption instead of summary in table

The summary attribute on the table element is obsolete.

grade/report/grader/lib.php

@@ -1265,7 +1265,8 @@ class grade_report_grader extends grade_report {
         $fulltable = new html_table();
         $fulltable->attributes['class'] = 'gradereport-grader-table';
         $fulltable->id = 'user-grades';
-        $fulltable->summary = get_string('summarygrader', 'gradereport_grader');
+        $fulltable->caption = get_string('summarygrader', 'gradereport_grader');
+        $fulltable->captionhide = true;
 
         // Extract rows from each side (left and right) and collate them into one row each
         foreach ($leftrows as $key => $row) {

lib/outputcomponents.php

@@ -2765,6 +2765,10 @@ class html_table {
 
     /**
      * @var string Description of the contents for screen readers.
+     *
+     * The "summary" attribute on the "table" element is not supported in HTML5.
+     * Consider describing the structure of the table in a "caption" element or in a "figure" element containing the table;
+     * or, simplify the structure of the table so that no description is needed.
      */
     public $summary;