Run PHPStan against PHP 7.1+

[n7studios]

Summary

1.9.0 and 1.9.0.1 shipped with Kit WordPress Libraries 2.1.4, which introduced a parse error on PHP ≤7.4 due to union types (PHP 8+ only) in the SDK:

syntax error, unexpected '|', expecting variable (T_VARIABLE)

1.9.1 was released, restoring the Kit WordPress Libraries back to 2.1.3. PHP 7.1+ compatibility was then restored in the Kit WordPress Libraries 2.1.5 via a backport.

This PR sets PHPStan’s phpVersion to ensure analysis targets PHP 7.1+ for the Plugin's code, to protect against a similar issue stemming from outside the Kit WordPress Libraries in the future.

While dropping older PHP support would be ideal, current usage indicates it’s not yet practical.

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)