Update kentik.mdx

[mbullock1986]

Updated Link that 404's to https://kb.kentik.com/v1/docs/mitigation-overview#cloudflare-mt-setup based on Kentik's PMs outreach to me.

Summary

Change URL to Kentik's support doc based on Kentik's PM's outreach.

Screenshots (optional)

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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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