Update botnet-threat-feed.mdx

[omer-cloudflare]

Updated the page to reflect that the DDoS Botnet Threat Feed is now generally available.

Summary

Screenshots (optional)

Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

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