Add better disable 2FA reference

What does this MR do?

Recently we have added a better example of how to disable 2FA via the Rails console via the TwoFactor::DestroyService in !88799 (merged) - https://docs.gitlab.com/ee/security/two_factor_authentication.html#for-a-single-user

The cheat sheet really should link to this.

We are also keeping the old Rails code for versions 13.4 and below because TwoFactor::DestroyService was introduced in 13.5. See !39572 (merged) for reference.

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