Fix migration for SSH key expiration enforcement

What does this MR do?

In !62432 (merged) we added a migration that is supposed to update the value of ApplicationSetting#enforce_ssh_key_expiration to true for all instances. However, that migration suffers from a bug that causes no ApplicationSetting to be updated for most instances. This fixes the bug in the migration.

The bug was deployed to Staging and Production, so the setting was manually updated to true for both of those environments by @djensen:

as = ApplicationSetting.first
as.enforce_ssh_key_expiration
=> false

as.enforce_ssh_key_expiration = true
as.save
=> true

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are 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.
• This change is backwards compatible across updates, or this does not apply.

Related to #320970 (closed)