Set Current Category to Active in category.php

You can filter the output on wp_list_categories:

add_filter( 'wp_list_categories', function( $html ) {
    return str_replace( ' current-cat', ' active', $html );
});

If you are stuck with an outdated PHP version … find a better web hosting. In the mean time, you can try this:

add_filter( 'wp_list_categories', 'replace_current_cat_css_class' );

function replace_current_cat_css_class( $html ) {
    return str_replace( ' current-cat', ' active', $html );
}