Change Creator Network Recommendations settings key

[n7studios]

Summary

Prevents a potential conflict with the main ConvertKit Plugin, which uses convertkit_creator_network_recommendations. A site with both this Plugin and the ConvertKit Plugin installed would otherwise share the setting, which might be undesirable if the user has two different ConvertKit accounts (unlikely, but never impossible).

Testing

Existing tests pass.

Checklist

• [x] I have written a test and included it in this PR
• [x] I have run all tests and they pass
• [x] The code passes when running the PHP CodeSniffer
• [x] Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• [x] Security and Sanitization requirements have been followed
• [x] 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)