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 https://github.com/kamermans/guzzle-oauth2-subscriber/commit/040db6816c9618841293d6eef05c88c244c2c550, 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.