PHP compatibility

Hi,

The plugin is marked as requiring PHP 7.0+ but the most recent updates (4.24.5 and 4.24.6) bricked a PHP 7.0 website with the following error:

PHP Parse error: syntax error, unexpected '?', expecting variable (T_VARIABLE) in /var/www/clients/client123/web123/web/wp-content/plugins/relevanssi/lib/common.php on line 1032

This is being caused by nullable parameter typing, a feature not introduced until PHP 7.1:

function relevanssi_is_front_page_id( ?int $post_id = null ): bool {

I’ve had to remove Relevanssi from the website, as other code on the site doesn’t work on PHP >7.0. Relevanssi needs to either declare support for PHP 7.1+ in its readme.txt, or not use nullable parameter typing.