Hi,
Updating to Joomla 4.4 won't fix the warning with your PHP 8.0 version being too old.
The updates of Joomla not working on PHP 8.1 on your website is not something we can help with. You need to ask directly on forum.joomla.org as we don't know what could be the problem.
Regarding the connection to PayPal not working on PHP 8.1, I don't know anything about it. But even if that's true, then it's not a problem for you since you were able to solve it by downgrading temporarily to PHP 8.0. Once the connection is established in the payment method, you don't need to do it again, and thus you can move back to PHP 8.1. And even if you can't, as I said in my previous message, you can circumvent the problem by manually entering the credentials.