Add column to store group ID in application_settings

What does this MR do?

Currently, the self-monitoring project is created by first creating a group, and then creating the self-monitoring project in that group. When the self-monitoring project is deleted, the group is also deleted since it was only used by the self-monitoring project.

The group is now required to be used for other features (#37964 (closed)). The ID of the group needs to be stored in DB so that it is easy to locate the group.

This MR adds a column in the application_settings table for storing the group ID.

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

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