Backport PHP 7.1+ Support

[n7studios]

Summary

#109 was included in 2.1.4 of the Kit WordPress Libraries, which resulted in errors (Intercom ticket) on sites running PHP 7.4 or lower:

An error of type E_PARSE was caused in line 147 of the file class-convertkit-api-traits.php. Error message: syntax error, unexpected '|', expecting variable (T_VARIABLE)"

This is due to union type declarations only being supported in PHP 8.0 or higher, and said trait file from the Kit PHP SDK only supporting PHP 8 and higher.

Kit Plugin updates were issued to resolve, rolling back the libraries to 2.1.3.

This wasn't caught by end to end or unit tests, as:

• they didn't run on PHP < 8.0,
• wp-coding-standards/wpcs only checks style (tabs vs spaces, naming conventions etc); not whether the syntax and functions are supported on a specific PHP version

Given that all Kit WordPress Plugins state a PHP supported version of 7.1 or higher, this PR resolves by:

• running coding tests and tests against PHP 7.2 and higher (WordPress 6.9.4 requires PHP 7.2 or higher)
• running tests against PHP 8.0 and higher (wp-browser and Codeception only support 8.0 and higher)
• configuring PHPStan's phpVersion, ensuring it runs code and static analysis against PHP 7.1 and higher
• replacing union type declarations to reintroduce compatibility with PHP 7.1 and higher

In the future, it would be good to drop support for older PHP versions - but given the number of errors caused, it seems many creators still use outdated PHP versions, and as such removing support doesn't seem feasible at this time.

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

[n7studios]

This is required for PHPStan static analysis to pass for PHP < 8.0, given the docblock comment supports different types.