Fix deprecation warnings with attribute #[\ReturnTypeWillChange]

[jvc9109]

Fixes deprecation warnings in PHP 8.1:

PHP Deprecated:  Return type of Infusionsoft\Api\Rest\RestModel::offsetExists($offset) should either be compatible with ArrayAccess::offsetExists(mixed $offset): bool, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in infusionsoft-php/src/Infusionsoft/Api/Rest/RestModel.php on line 985
PHP Deprecated:  Return type of Infusionsoft\Api\Rest\RestModel::offsetGet($offset) should either be compatible with ArrayAccess::offsetGet(mixed $offset): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in infusionsoft-php/src/Infusionsoft/Api/Rest/RestModel.php on line 997
PHP Deprecated:  Return type of Infusionsoft\Api\Rest\RestModel::offsetSet($offset, $value) should either be compatible with ArrayAccess::offsetSet(mixed $offset, mixed $value): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in infusionsoft-php/src/Infusionsoft/Api/Rest/RestModel.php on line 1010
PHP Deprecated:  Return type of Infusionsoft\Api\Rest\RestModel::offsetUnset($offset) should either be compatible with ArrayAccess::offsetUnset(mixed $offset): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in infusionsoft-php/src/Infusionsoft/Api/Rest/RestModel.php on line 1022
PHP Deprecated:  Return type of Infusionsoft\Api\Rest\RestModel::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in infusionsoft-php/src/Infusionsoft/Api/Rest/RestModel.php on line 574
PHP Deprecated:  Return type of Infusionsoft\InfusionsoftCollection::offsetExists($key) should either be compatible with ArrayAccess::offsetExists(mixed $offset): bool, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in infusionsoft-php/src/Infusionsoft/InfusionsoftCollection.php on line 147
PHP Deprecated:  Return type of Infusionsoft\InfusionsoftCollection::offsetGet($key) should either be compatible with ArrayAccess::offsetGet(mixed $offset): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in infusionsoft-php/src/Infusionsoft/InfusionsoftCollection.php on line 158
PHP Deprecated:  Return type of Infusionsoft\InfusionsoftCollection::offsetSet($key, $value) should either be compatible with ArrayAccess::offsetSet(mixed $offset, mixed $value): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in infusionsoft-php/src/Infusionsoft/InfusionsoftCollection.php on line 170
PHP Deprecated:  Return type of Infusionsoft\InfusionsoftCollection::offsetUnset($key) should either be compatible with ArrayAccess::offsetUnset(mixed $offset): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in infusionsoft-php/src/Infusionsoft/InfusionsoftCollection.php on line 185
PHP Deprecated:  Return type of Infusionsoft\InfusionsoftCollection::count() should either be compatible with Countable::count(): int, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in infusionsoft-php/src/Infusionsoft/InfusionsoftCollection.php on line 136
PHP Deprecated:  Return type of Infusionsoft\InfusionsoftCollection::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in infusionsoft-php/src/Infusionsoft/InfusionsoftCollection.php on line 115