Regression in 11.5.3.1

Hello,

my 6.1.6 WordPress now has a fatal error since upgrading ICS Calendar 11.5.3.1, because of the list_files function:

PHP Fatal error:  Uncaught Error: Call to undefined function list_files() in /var/lib/wordpress/wp-content/plugins/ics-calendar/ics-calendar.php:102
Stack trace:
#0 /var/lib/wordpress/wp-content/plugins/ics-calendar/ics-calendar.php(86): r34ics_i18n_locales()
#1 /usr/share/wordpress/wp-includes/class-wp-hook.php(308): r34ics_load_textdomain_mofile()
#2 /usr/share/wordpress/wp-includes/plugin.php(205): WP_Hook->apply_filters()
#3 /usr/share/wordpress/wp-includes/l10n.php(759): apply_filters()
#4 /usr/share/wordpress/wp-includes/l10n.php(924): load_textdomain()
#5 /var/lib/wordpress/wp-content/plugins/ics-calendar/ics-calendar.php(72): load_plugin_textdomain()
#6 /usr/share/wordpress/wp-includes/class-wp-hook.php(308): r34ics_load_plugin_textdomain()
#7 /usr/share/wordpress/wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters()
#8 /usr/share/wordpress/wp-includes/plugin.php(517): WP_Hook->do_action()
#9 /usr/share/wordpress/wp-settings.php(617): do_action()
#10 /usr/share/wordpress/wp-config.php(71): require_once('...')
#11 /usr/share/wordpress/wp-load.php(50): require_once('...')
#12 /usr/share/wordpress/wp-admin/admin.php(34): require_once('...')
#13 /usr/share/wordpress/wp-admin/index.php(10): require_once('...')
#14 {main}
  thrown in /var/lib/wordpress/wp-content/plugins/ics-calendar/ics-calendar.php on line 102

This was introduced in 11.5.3.1: https://plugins.trac.www.ads-software.com/changeset?sfp_email=&sfph_mail=&reponame=&new=3236243%40ics-calendar%2Ftrunk%2Fics-calendar.php&old=3234220%40ics-calendar%2Ftrunk%2Fics-calendar.php&sfp_email=&sfph_mail=