Add secret detection rule for Age private keys

What does this MR do?

This MR adds a new secret detection rule to detect Age private keys. Age is a file encryption tool that is intended to be a modern replacement for PGP. It has asymmetric encryption keys that are uniquely identifiable with the AGE-SECRET-KEY prefix, and use a Bech32 encoding.

I've tested this regex against 10 newly generated private keys: https://regex101.com/r/8hAgyj/1/

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
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• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer