Add PHP 8.2 support.

[mikkamp]

Changes proposed in this Pull Request:

This PR fixes some deprecation messages when run on a site with PHP 8.2. This was mostly resolved by declaring the class properties for all the settings.

In addition to that the dismissed_info_banner property was removed as it was no longer present and had been removed in PR #260

I've also modified the phpunit workflow to test with newer PHP versions including PHP 8.2.

Note
The workflow change will conflict with PR #290 when it's merged, but this should be relatively straightforward to merge.

Closes #291

Detailed test instructions:

1. Test extension on a site with PHP 8.2 and WP_DEBUG enabled
2. Setup some basic settings
3. Purchase a product (in an incognito window)
4. Confirm that tracking events are being recorded
5. Ensure there aren't any warnings / deprecation messages showing up at any stage
6. Check the automated PR tests and confirm they ran for PHP 8.2 without errors

Changelog entry

• Compat - Add PHP 8.2 support.

[jorgemd24]

Thanks @mikkamp for working on this. LGTM 👍

✅ I've compared the PHP Unit tests using both PHP 8.0 and PHP 8.2, and the deprecation notices have been resolved.

✅ Additionally, I've verified that the tracking events are being properly recorded.