Follow-up of https://github.com/FreshRSS/FreshRSS/pull/2630 now that we have PHP 7.3+ (even PHP 8.1+).
* The new syntax natively supports `samesite`, and also avoids the need of re-setting all parameters.
* Use automatic path instead of own function `getCookieDir()`.
Follow-up of https://github.com/FreshRSS/FreshRSS/pull/8446
* Sanitize lifetime of session cookies from PHP ini to avoid likely invalid/misunderstood values