Use new static self in s3 plugin

In addition, this best practice is instructed to PHPStan via https://git.drupalcode.org/project/drupal/-/blob/11.x/core/phpstan.neon....

After updating `flysystem_s3`, one of our custom modules which extends Drupal\flysystem_s3\Flysystem\S3 starting throwing some of the "deprecated: dynamic property" warnings due trying to set against an instance of the upstream class instead of the static bound class.

MR from #2 (at e9995f3f8a1db1f2aef0d37ccb7dce0a7184406b) seems to do the trick to get rid of the warning (and restore the behaviours from the extending code).