Fix PHP8.1 tentative return types

[ChristophWurst]

PHP Deprecated:  Return type of Horde_Support_CaseInsensitiveArray::offsetExists($offset) should either be compatible with ArrayIterator::offsetExists(mixed $key): bool, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /home/runner/work/Imap_Client/Imap_Client/vendor/bytestream/horde-support/lib/Horde/Support/CaseInsensitiveArray.php on line 47
PHP Deprecated:  Return type of Horde_Support_CaseInsensitiveArray::offsetGet($offset) should either be compatible with ArrayIterator::offsetGet(mixed $key): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /home/runner/work/Imap_Client/Imap_Client/vendor/bytestream/horde-support/lib/Horde/Support/CaseInsensitiveArray.php on line 27
PHP Deprecated:  Return type of Horde_Support_CaseInsensitiveArray::offsetSet($offset, $value) should either be compatible with ArrayIterator::offsetSet(mixed $key, mixed $value): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /home/runner/work/Imap_Client/Imap_Client/vendor/bytestream/horde-support/lib/Horde/Support/CaseInsensitiveArray.php on line 36
PHP Deprecated:  Return type of Horde_Support_CaseInsensitiveArray::offsetUnset($offset) should either be compatible with ArrayIterator::offsetUnset(mixed $key): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /home/runner/work/Imap_Client/Imap_Client/vendor/bytestream/horde-support/lib/Horde/Support/CaseInsensitiveArray.php on line 54
PHP Deprecated:  Return type of Horde_Support_Stub::count() should either be compatible with Countable::count(): int, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /home/runner/work/Imap_Client/Imap_Client/vendor/bytestream/horde-support/lib/Horde/Support/Stub.php on line 126
PHP Deprecated:  Return type of Horde_Support_Stub::getIterator() should either be compatible with IteratorAggregate::getIterator(): Traversable, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /home/runner/work/Imap_Client/Imap_Client/vendor/bytestream/horde-support/lib/Horde/Support/Stub.php on line 135
PHP Deprecated:  Return type of Horde_Support_Stub::offsetExists($offset) should either be compatible with ArrayAccess::offsetExists(mixed $offset): bool, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /home/runner/work/Imap_Client/Imap_Client/vendor/bytestream/horde-support/lib/Horde/Support/Stub.php on line 111
PHP Deprecated:  Return type of Horde_Support_Stub::offsetGet($offset) should either be compatible with ArrayAccess::offsetGet(mixed $offset): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /home/runner/work/Imap_Client/Imap_Client/vendor/bytestream/horde-support/lib/Horde/Support/Stub.php on line 98
PHP Deprecated:  Return type of Horde_Support_Stub::offsetSet($offset, $value) should either be compatible with ArrayAccess::offsetSet(mixed $offset, mixed $value): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /home/runner/work/Imap_Client/Imap_Client/vendor/bytestream/horde-support/lib/Horde/Support/Stub.php on line 105
PHP Deprecated:  Return type of Horde_Support_Stub::offsetUnset($offset) should either be compatible with ArrayAccess::offsetUnset(mixed $offset): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /home/runner/work/Imap_Client/Imap_Client/vendor/bytestream/horde-support/lib/Horde/Support/Stub.php on line 118

Upstream PR: horde#2

[ChristophWurst]

cc @bytestream