Revert temporary change for sending expiration email for all SSH keys

What does this MR do and why?

1. We started sending SSH key expiration notification emails for all expired keys in the MR !62114 (merged). This change is no longer needed and, therefore, changing back to sending notifications only for the keys that are expiring today.
2. Also removing the index that is not required anymore. The queries will now use the index index_keys_on_expires_at_and_id.

Database

Migration

Up

== 20211027112901 DropIndexKeysOnExpiresAtAndBeforeExpiryNotificationUndelivered: migrating
-- transaction_open?()
   -> 0.0000s
-- indexes(:keys)
   -> 0.0049s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- remove_index(:keys, {:algorithm=>:concurrently, :name=>"index_keys_on_expires_at_and_expiry_notification_undelivered"})
   -> 0.0042s
-- execute("RESET statement_timeout")
   -> 0.0006s
== 20211027112901 DropIndexKeysOnExpiresAtAndBeforeExpiryNotificationUndelivered: migrated (0.0329s)

Down

== 20211027112901 DropIndexKeysOnExpiresAtAndBeforeExpiryNotificationUndelivered: reverting
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:keys, "date(timezone('UTC', expires_at)), expiry_notification_delivered_at", {:where=>"expiry_notification_delivered_at IS NULL", :name=>"index_keys_on_expires_at_and_expiry_notification_undelivered", :algorithm=>:concurrently})
   -> 0.0038s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- add_index(:keys, "date(timezone('UTC', expires_at)), expiry_notification_delivered_at", {:where=>"expiry_notification_delivered_at IS NULL", :name=>"index_keys_on_expires_at_and_expiry_notification_undelivered", :algorithm=>:concurrently})
   -> 0.0026s
-- execute("RESET statement_timeout")
   -> 0.0006s
== 20211027112901 DropIndexKeysOnExpiresAtAndBeforeExpiryNotificationUndelivered: reverted (0.0142s)

Query Plan

explain SELECT "keys".* FROM "keys" WHERE (date(expires_at AT TIME ZONE 'UTC') = CURRENT_DATE AND expiry_notification_delivered_at IS NULL) ORDER BY date(expires_at AT TIME ZONE 'UTC') ASC, "keys"."id" ASC

 Index Scan using index_keys_on_expires_at_and_id on public.keys  (cost=0.43..36717.41 rows=24797 width=638) (actual time=0.025..0.026 rows=0 loops=1)
   Index Cond: (date(timezone('UTC'::text, keys.expires_at)) = CURRENT_DATE)
   Buffers: shared hit=4
   I/O Timings: read=0.000 write=0.000

Time: 0.333 ms
  - planning: 0.276 ms
  - execution: 0.057 ms
    - I/O read: 0.000 ms
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 4 (~32.00 KiB) from the buffer pool
  - reads: 0 from the OS file cache, including disk I/O
  - dirtied: 0
  - writes: 0

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Related to #331927 (closed)