[SSH keys] Revert temporary "all records" change to expiration notification emails

Problem to solve

In %13.12 we modified SSH key expiration notification emails to consider all SSH keys, not just those expiring "today". This will cause one large batch of emails to be sent, but is no longer necessary after that batch is sent.

Proposal

After a reasonable period of time to allow for the email batch to be sent, the prior changes should be reverted:

1. The worker should only consider keys expiring "today".
2. The database indexes should be removed.

• index_keys_on_expires_at_and_expiry_notification_undelivered
• idx_keys_expires_at_and_before_expiry_notification_undelivered