(Part 1) Resolve "Recognise when a user is trying to validate a private SSH key"

What does this MR do?

Plan: https://gitlab.com/snippets/1723890

Updates the new SSH key page (profile/keys) to improve the copy to better explain to the user what is required.

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

Why was this MR needed?

The new SSH key page could be confusing, some users enter a private key instead of a public key.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Part of #46396 (closed)