Set session cookies to strict (#3983)

#3982

src/deb/php/php-fpm.conf

@@ -35,6 +35,7 @@ php_admin_flag[log_errors] = on
 php_admin_flag[session.cookie_httponly] = on
 php_admin_flag[session.use_strict_mode] = on
 php_admin_flag[session.cookie_secure] = on
+php_admin_value[session.cookie_samesite] = "Strict"
 php_admin_value[memory_limit] = 256M
 php_admin_value[post_max_size] = 256M
 php_admin_value[upload_max_filesize] = 256M