Fix(New Subscription): handle non-existing groups

What does this MR do?

This MR handles when the user tries to upgrade a group that does not exist or they do not have access to it, e.g. when a link is shared and the group was deleted in the meantime or when the sharer has access, but the receiver doesn't.

Screenshots (strongly suggested)

Before:	After:

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

To test this out:

• Navigate to this URL and see it load: http://localhost:3000/-/subscriptions/new?namespace_id=28&plan_id=2c92c0f95a24621b015a259a50307881
• Update the URL to some namspace that does not exists, e.g. 281: http://localhost:3000/-/subscriptions/new?namespace_id=281&plan_id=2c92c0f95a24621b015a259a50307881
  • Before the change it would fail
  • After the change it would still load the page and just display the groups that are available

• 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

Related to: customers-gitlab-com#2200 (comment 451662041)