Big problem after update PHP form 8.0 to 8.1

[jjawatches]

After updating PHP, the website does not work properly. Payment cannot be made and many other problems (photo number 4).
For example:
In the Phoca Cart panel, in the "Payment" section, after clicking on the selected payment method (photo no. 1), the "Title *" field is empty (photo no. 2).
Even if I fill in "debit/credit card" in this field and click "save", an error appears (photo no. 3)
It is the same in every other section, e.g. order statuses, coupons, discounts, shipping, etc.
It is similar when I click on the customer (customers section) Photo no. 5
It looks like some fields are missing in the database.
Can this be fixed or do I have to install new Joomla and start all over?
Unfortunately, I cannot go back to PHP 8.0
Joomla version: 3.10.2
Phoca Cart version:3.5.8
Thank you
Pawel

https://jjawatches.co.uk/images/php/1.jpg
https://jjawatches.co.uk/images/php/2.jpg
https://jjawatches.co.uk/images/php/3.jpg
https://jjawatches.co.uk/images/php/4.jpg
https://jjawatches.co.uk/images/php/5.jpg

[jjawatches]

I just noticed that this is not just a problem with Phoca Cart. All components, modules and plugins are not working.

[Nidzo]

You must update Joomla to version 4 or 5.
Joomla 3 does not work well with PHP 8.1 as you can see.