x/website: newer examples for security policy PRIVATE issues

[seankhliao]

The security policy page https://go.dev/security/policy lists examples of PRIVATE issues.
These are examples listed in the new security policy discussion #44918 , before the policy was in place.

I've witnessed confusion on what PRIVATE means, as some of the issues listed there were open for quite some time before being fixed.

Newer PRIVATE issues follow a different process with placeholder issues linking to a private tracker that is updated when it becomes public, ex: #53416 #53616 #54658 #56284 .

The security policy would benefit from newer examples that follow the process laid out by it.

cc @golang/security

[gopherbot]

Change https://go.dev/cl/460055 mentions this issue: _content/security: update PRIVATE track examples