Docs: Rename feature - Secret push protection

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What does this MR do?

Removes references to pre-receive secret detection and replaces them with secret push protection.

Note: The feature name "pre-receive secret detection" has not yet been renamed in the UI. That will be addressed in the MR in which the UI elements are renamed.

Related issues

• Update feature name - Documentation (#462290)
• Rename to Secret Push Protection (#462289)

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