Remove ability for SSH key expiration to be optional

What does this MR do and why?

• Removes the ability for instance administrators to unenforce SSH key expiration.
• Removes the ability for any user to use an expired SSH key for git actions.
• Expired SSH keys are no longer valid.

How to set up and validate locally

• Add an SSH key to your user account and manually adjust the expiration to the past using the rails console.

key.update_attribute(:expires_at, 3.days.ago)

• Attempt to use that key to clone, push or pull.
• Note the error: remote: ERROR: Your SSH key has expired.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Reviewers needed

• backend reviewer
• backend maintainer
• frontend reviewer
• frontend maintainer
• documentation

• I have evaluated the MR acceptance checklist for this MR.

Related to #351963 (closed)