version check on branch is broken

Version check on branch is actually broken. But it has not been noticed since we have not had any updates since it was implemented.

But the new get_update_on_branch() method actually returns a 1-dimensional array, not a multi-dimensional array as does get_suggested_updates(), so it is not being passed to the template correctly, resulting in

PHP Fatal error: Uncaught TypeError: Argument 2 passed to phpbb\template\base::assign_block_vars() must be of the type array, string given, called in /phpBB/includes/acp/acp_update.php on line 58 and defined in /phpBB/phpbb/template/base.php:110