SSH key expiration notification email

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

Send an email to the 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 that same email template should be used.