docs: update security url to link to central policy

[ismisepaul]

This PR replaces security URLs with a link to the central security policy.

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🔍 Preview links for changed docs

• docs/reference/frequently-asked-questions.md

[jackshirazi]

why is that better than the URL?

[ismisepaul]

The URL is subject to change it recently went from elastic.co/community/security to elastic.co/product-security. There could be future changes like calling it PSIRT whereas https://github.com/elastic/.github/blob/main/SECURITY.md will remain constant

[jackshirazi]

yes, and we put a forwarder in place so the old URL forwards to the new one. The UX is much better to go directly to the security page. If we change it to the github page, you give the customer a worse experience - not only do they have to do the additional onward click, but they also have the bizarre combination of "here, I've just redirected you to a github page where I'm telling you not to use github for security issues ... and additionally I'm pointing you at the real security page instead of giving you any useful info here even though I've just made you come here"

[ismisepaul]

We can keep the URL to the policy instead of the github file.