In-app SSH key expiration notification

Problem to solve

Under #250480 (closed) we implemented the ability for administrators to toggle SSH key expiration enforcement. Enforcement is disabled by default, but under #320970 (closed) we plan to make it enabled by default. That will be a breaking change, because it will cause any expired SSH keys to immediately stop being functional. We should actively communicate this breaking change in advance.

Proposal

Show an in-app notification for a user when the user has any SSH keys which:

• are already expired.
• will expire within 7 days.

This is the same behavior that already exists for Personal Access Tokens, and the same notification language should be used.