Add accessibility to the menu on the navbar (go-gitea#23059)

This PR is trying to add accessibility to the menu as mentioned in
go-gitea#23053 so the menu can be accessed using keyboard (A quick demo is added
below), with a reference to
[PR2612](go-gitea#22612). The goal is to
make the menu accessible merely using keyboard like shown below. And
this PR might need confirmation from developers using screen readers.

templates/base/head_navbar.tmpl

@@ -18,9 +18,9 @@
 				</span>
 			</a>
 			{{end}}
-			<div class="ui icon button mobile-only" id="navbar-expand-toggle">
+			<button class="ui icon button mobile-only" id="navbar-expand-toggle">
 				{{svg "octicon-three-bars"}}
-			</div>
+			</button>
 		</div>
 	</div>