Add migration to save Instance Administrators group ID

What does this MR do?

When the self monitoring project is created, its ID is saved into the application_settings table. The project is created within a group called the Instance Administrators group. Previously, the group ID was not saved in application_settings. Recently, we started saving the Instance Administrators group ID into the application_settings table when the group is created (the group is created along with the project).

If the self monitoring project has already been created, the Instance Administrators group will also have been created, but not saved into application_settings.

This MR reads the owner of the self monitoring project and saves it into the application_settings table as the Instance Administrators group.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] 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