Rename secret "post-processing" to "automatic response"

What does this MR do?

Update the title and introductory content of the "Post-processing and revocation" page for Category:Secret Detection.

Thoughts behind this:

• This term is familiar to us, but it describes more of what the system is doing versus what the actual user-facing result is: they leak a secret and we immediately respond. It seems odd to expect users to know what "post-processing" means.
• "Revocation" is not actually the response that is done for all partners. Some partners apply different kinds of mitigations rather than immediately revoking the secret.

This docs change mirrors a previous change to the marketing site: gitlab-com/www-gitlab-com!119911 (merged).

I have not yet:

• Updated the page path. This way we don't have to immediately figure out redirects or update links everywhere. This would be particularly troublesome while a release post is being prepared.
• Changed a link from the main Secret Detection page. That link is edited separately in !118380 (merged).

Related issues

Update description and name of automatic Secret... (gitlab-com/www-gitlab-com!119911 - merged)

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