Prevent conversion if any domain uses PHP provided by os vendor

[SandakovMM]

Since vendor version of PHP is 5.4 on CentOS 7 we should force to move from it before the conversion.
We cannot directly upgrade php-handler to the PHP 7.2 provided by the vendor on AlmaLinux 8. This is because there is a risk that the website will break due to PHP version incompatibility.
Additionally, staying on PHP 5.4 is not an option since it is not provided by the vendor or Plesk on AlmaLinux 8.
As a solution, we need to prevent the upgrade and advise a customer to switch to PHP 7.2 provided by Plesk or any other modern PHP version.
It would be beneficial to implement a pre-conversion checker for this issue.

[SandakovMM]

Problem should be fixed in #201