Implement PHPStan level 8 compliance for `script-loader.php` #6545

Job	Run time
Build / WordPress	2m 20s
Upgrade from 6.7 / PHP 8.4 with MySQL 8.4	1m 43s
Upgrade from 6.9-RC1 / PHP 8.4 with MySQL 5.7	1m 29s
Upgrade from 6.9-RC1 / PHP 8.4 with MySQL 8.4 multisite	1m 15s
Upgrade from 4.9 / PHP 7.2 with MySQL 5.7 multisite	1m 8s
Upgrade from 6.8 / PHP 7.2 with MySQL 5.7 multisite	1m 35s
Upgrade from 6.8 / PHP 7.2 with MySQL 5.7	1m 38s
Upgrade from 6.8 / PHP 8.4 with MySQL 8.4	1m 32s
Upgrade from 6.8 / PHP 8.4 with MySQL 5.7 multisite	1m 36s
Upgrade from 6.7 / PHP 8.4 with MySQL 8.4 multisite	1m 34s
Upgrade from 6.7 / PHP 8.4 with MySQL 5.7	1m 34s
Upgrade from 6.9-RC1 / PHP 7.2 with MySQL 5.7	1m 2s
Upgrade from 6.9-RC1 / PHP 8.4 with MySQL 5.7 multisite	1m 30s
Upgrade from 6.8 / PHP 8.4 with MySQL 5.7	1m 34s
Upgrade from 6.7 / PHP 7.2 with MySQL 5.7	1m 41s
Upgrade from 6.9-RC1 / PHP 7.2 with MySQL 5.7 multisite	1m 1s
Upgrade from 6.7 / PHP 8.4 with MySQL 5.7 multisite	1m 31s
Upgrade from 4.9 / PHP 7.2 with MySQL 5.7	1m 39s
Upgrade from 6.8 / PHP 8.4 with MySQL 8.4 multisite	1m 16s
Upgrade from 6.7 / PHP 7.2 with MySQL 5.7 multisite	1m 37s
Upgrade from 6.9-RC1 / PHP 8.4 with MySQL 8.4	1m 41s
Upgrade from ${{ matrix.wp }}	0s
Slack Notifications	0s
Failed workflow tasks	0s
	31m 56s

Provide feedback