Support PHP 8

[mikejpeters]

Currently this project requires php ^7.1, which causes composer errors if this library is used in a project targeting PHP 8:

Your requirements could not be resolved to an installable set of packages.

Problem 1
- petfinder-com/petfinder-php is locked to version v1.0.2 and an update of this package was not requested.
- petfinder-com/petfinder-php v1.0.2 requires php ^7.1 -> your php version (8.0; overridden via config.platform, actual: 8.0.11) does not satisfy that requirement.

Can you update the library to support PHP 8? Thank you!

[sanjay-possibility]

[InvalidArgumentException]
Package petfinder-com/petfinder-php has a PHP requirement incompatible with your PHP version, PHP extensions and Composer version:
- petfinder-com/petfinder-php v1.0.2 requires php ^7.1 which does not match your installed version 8.0.14.