Remove v3 API Key and Secret on Plugin Update

[n7studios]

Summary

Whenever a new Plugin release is published and the creator updates to the latest version, checks if a v3 API Key and/or Secret exists, and if so deletes it from the Plugin's settings.

v3 API Keys and Secrets were retained following the introduction of the v4 API, to permit automatic exchange of v3 API Keys for v4 tokens. This was in July 2024, so by now the v3 API Key and Secret isn't needed.

Testing

• testV3APIKeyAndSecretRemovedFromSettings: Test that a v3 API Key and Secret are removed from the Plugin's settings when specified.

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)