Do not delete instance_administrators_group

What does this MR do?

When creating the self-monitoring project, an "Instance Administrators" group is first created and the self-monitoring project is created within that group. Also, when the self-monitoring project is deleted, the group is deleted along with it.

Since we now plan to use the "Instance Administrators" group for other purposes in addition to the self-monitoring project, this MR changes the behavior of the SelfMonitoring::Project::DeleteService to not delete the "Instance Administrators" group.

Issue: #37964 (closed)

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