Remove obsolete upgrade note (#7138)

|      Q       |   A
|------------- | -----------
| Type         | documentation
| Fixed issues | N/A

#### Summary

The workaround documented in this upgrade note has been reverted in
#6623.

Author: derrabus

UPGRADE.md

@@ -398,10 +398,7 @@ and `::dropUniqueConstraint()` respectively instead.
 ## Support for new PDO subclasses on PHP 8.4
 
 On PHP 8.4, if you call `getNativeConnection()` on a connection established through one of the PDO drivers,
-you will get an instance of the new PDO subclasses, e.g. `Pdo\Mysql` or `Pdo\Ppgsql` instead of just `PDO`.
-
-However, this currently does not apply to persistent connections.
-See https://github.com/php/php-src/issues/16314 for details.
+you will get an instance of the new PDO subclasses, e.g. `Pdo\Mysql` or `Pdo\Pgsql` instead of just `PDO`.
 
 ## Minor BC break: incompatible query cache format