Deprecation notice about return type of several methods of the Collection class

[cyberwolf]

On PHP 8.1, deprecation notices still occur on several methods of the Util\sCollection class.

#[ReturnTypeWillChange] was added to the offsetGet() method in 040db68, however it was not added to any of the other methods that are affected, e.g. offsetExists().

Ideally all methods of ArrayAccess, IteratorAggregate and Countable should be verified.