Fix deprecation on PHP

[xelan]

Q	A
Branch?	2.x
Bug fix?	yes
New feature?	no
Deprecations?	no
Tickets	Fix #345
License	MIT

This PR fixes the deprecation message occuring on an OOTB Symfony 5.4 on PHP 7.4:

User Deprecated: Method "IteratorAggregate::getIterator()" might add "\Traversable" as a native return type declaration in the future. Do the same in implementation "Eluceo\iCal\Presentation\Component" now to avoid errors or add an explicit @return annotation to suppress this message.

The other issues outlined in #369 should already be solved, as the #[ReturnTypeWillChange] annotations are already present in those classes.

Thank you very much for the awesome library and best regards from Austria
Andreas

[markuspoerschke]

Hallo Andreas (@xelan), vielen Dank!

I adjusted the PR a little bit:

1. I added a note to the changelog
2. I converted the PHPDocBlock return type hint to a native return type of the method

[xelan]

Thank you very much for merging 👍 Bitte, gerne 😉