Update description and name of automatic Secret Detection response feature

Why is this change being made?

The description of Secret Detection Post-Processing was out-of-date. It also didn't reflect the current limitations on the tiers in which post-processing is available.

Reviewer note: this feature description is accessible on the website at https://about.gitlab.com/features/?stage=secure#secret_detection. Review app link showing changes: https://connorgilbert-features-sd-revocation.about.gitlab-review.app/features/?stage=secure#secret_detection

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