fix(ui5-li-groupheader): use group role on UL nodes only (#5518)

Fixes: #5400

Author: dobrinyonkov

packages/main/src/GroupHeaderListItem.hbs

@@ -1,4 +1,4 @@
-<li
+<ul
 	part="native-li"
 	tabindex="{{_tabIndex}}"
 	class="ui5-ghli-root {{classes.main}}"
@@ -12,4 +12,4 @@
 	<div id="{{_id}}-content" class="ui5-li-content">
 		<span class="ui5-ghli-title"><slot></slot></span>
 	</div>
-</li>
+</ul>