Increase minimum required version of PHP to 7.1 (up from 7.0).

[barryhughes]

We are increasing the minimum required version of PHP. Functionally, nothing has changed (but we will take advantage of this in some upcoming PRs, to address problems that occur when using PHP 8.4).

Closes #1211.

🥼 Testing

1. As noted above, functionally, nothing has changed. As a minimum, we still expect the test suite to pass without errors.
2. A high level review or 'smoketest' of the plugin is probably useful:
   1. Activate the plugin (as a top-level, standalone plugin).
   2. Ensure the Tools ‣ Scheduled Actions admin screen is still present and functions as normal.
3. Feel free to do any other exploratory testing you feel may be useful.

📝 Changelog

Bump minimum PHP version to 7.1.

[barryhughes]

Once/if we are happy with this change, we can go ahead and update the base branch for #1205, then hopefully get that worked reviewed and merged. This will also be useful in getting projects like WooCommerce ready for PHP 8.4.