Add #[\ReturnTypeWillChange] for PHP 8.1

[luigifab]

This PR fix with PHP 8.1/8.2:

Return type of Adamlc\AddressFormat\Format::offsetExists($offset) should either be compatible
 with ArrayAccess::offsetExists(mixed $offset): bool, or the #[\ReturnTypeWillChange] attribute
 should be used to temporarily suppress the notice

[adamlc]

Having not worked with PHP for some time, will this break on older versions?

[luigifab]

No, it's a comment, so it doesn't break anything... except if someone create a class named ReturnTypeWillChange. The RFC page only mention PHP 7. I checked with PHP 5.0..8.2: simple example.

[adamlc]

Fantastic appreciate the testing :)