Speed up SSH key expiration notification queries

What does this MR do?

• Changes the batch size when finding users with expired SSH keys which seems to alter the query plan created by postgres and executes much faster.
• This is not the most elegant solution although it does work fairly consistently. I'm open to better ideas to achieve the same result.

Query Plans

Before	After
https://explain.depesz.com/s/Kdmq	https://explain.depesz.com/s/f7sXy

Query

SELECT
        "users" . *
    FROM
        "users"
    WHERE
        (
            EXISTS (
                SELECT
                        1
                    FROM
                        "keys"
                    WHERE
                        (
                            keys.user_id = users.id
                        )
                        AND (
                            DATE (
                                expires_at AT TIME ZONE 'UTC'
                            ) = CURRENT_DATE
                            AND expiry_notification_delivered_at IS NULL
                        )
            )
        )
    ORDER BY
        "users" . "id" ASC LIMIT 10000

Database Lab

Before

Time: 15.078 s  
  - planning: 0.788 ms  
  - execution: 15.077 s (estimated* for prod: 9.379...14.559 s)  
    - I/O read: 18.228 s  
    - I/O write: N/A  
  
Shared buffers:  
  - hits: 7737502 (~59.00 GiB) from the buffer pool  
  - reads: 566323 (~4.30 GiB) from the OS file cache, including disk I/O  
  - dirtied: 53345 (~416.80 MiB)  
  - writes: 0  

After

Time: 23.413 ms  
  - planning: 6.808 ms  
  - execution: 16.605 ms  
    - I/O read: 14.408 ms  
    - I/O write: N/A  
  
Shared buffers:  
  - hits: 85 (~680.00 KiB) from the buffer pool  
  - reads: 221 (~1.70 MiB) from the OS file cache, including disk I/O  
  - dirtied: 2 (~16.00 KiB)  
  - writes: 0  

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #331101 (closed)