Replace deprecated `PDO::MYSQL_*` constant usage with the driver-specific
`Pdo\Mysql::ATTR_*` constants introduced in PHP 8.4.
This prepares the code for PHP 8.5, where accessing MySQL driver constants
through the generic `PDO` class is deprecated.
This change requires a compatibility shim on older PHP versions to provide
`Pdo\Mysql` for runtimes that do not expose the driver-specific class yet. The
shim is provided in `ipl-sql`. As a consequence, the required version of the
Icinga PHP library has been raised.
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