assertArrayHasKey(), assertArrayNotHasKey(), arrayHasKey(), and ArrayHasKey::__construct() do not support all possible key types

[vudaltsov]

ArrayAccess can use mixed keys (see https://www.php.net/manual/en/class.arrayaccess.php), however, Assert::assertArrayHasKey(), Assert::assertArrayNotHasKey() accept only int|string keys, which does not allow to test collections with object keys.

I propose to change int|string $key to mixed $key. This will not break BC, since the methods are final.