Update yahnis-elsts/plugin-update-checker from 5.1 to 5.7

[peterwilsoncc]

Description of the Change

The Plugin Update Checker library we use is also used on a few other projects. We need to ensure we are using the same version across all of these or you can end up with conflicts (especially when using composer to manage all dependencies of a project). This PR updates this library to the latest version, which will also be done in other repos that are using this library.

How to test the Change

1. Pull these changes down
2. Within the distributor.php file, set the version number to something lower, like 2.1.0
3. Activate Distributor if required.
4. Ensure you have a proper license in place
5. Go to the plugins section and ensure you see an update notice for Distributor (may need to click the Check for updates link)
6. Click on the View version X.X.X details link and ensure you see proper details
7. Go to the WordPress Updates page and ensure you see Distributor there as well, with the icon
8. Click on the View version X.X.X details link and ensure you see proper details
9. If desired, click the update now link and ensure an update works (either from the plugins page or the main updates page). Note though that this will wipe out your version controlled distributor directory, so proceed with caution

Changelog Entry

Changed - Upgrade the Plugin Update Checker library from 5.1 to 5.7

Credits

Props @peterwilsoncc

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.