Spam Protection: Add Cloudflare Turnstile

[n7studios]

Summary

Adds Cloudflare Turnstile as a spam protection option for the Form Builder and Member Content functionality, as Google's reCAPTCHA v3 is replaced with https://docs.cloud.google.com/recaptcha/docs, which has costs above 10,000 submissions/month.

reCAPTCHA v3 is retained as existing site keys still work with this, but new ones cannot be registered.

Testing

• testCloudflareTurnstileSettings
• testFormBuilderWithCloudflareTurnstileEnabled
• testRestrictContentByTagWithCloudflareTurnstileEnabled

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)

[github-actions]

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