Show only root namespaces for subscriptions

What does this MR do?

Part of #229104 (closed)

We only allow subscriptions for root namespaces. However, we also rendered sub-groups when creating a subscription. This was not an issue so far, because we only used the new subscription sites for sign up, therefore users were not able to create sub-groups beforehand.

This will change in the future, where we also want to use the new subscription page design for upgrades.

To not make it possible to create a subscription for sub groups in the future, we will introduce a validation

Screenshots

Before	After (subgroup does not appear anymore)

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