Ensure key fingerprints are generated correctly when modified

What does this MR do?

Backports a commit from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3012

When implementing https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13712 , I believed there were no cases in which Key#key would be mutated - however, we are now adding a case that does exactly that.

When the key is mutated, a memoized value must be flushed or the key will be generated with an invalid fingerprint.

Are there points in the code the reviewer needs to double check?

Would it be better to remove the memoization entirely?

Why was this MR needed?

Avoid divergence between CE and EE

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?