Enforce presence of Value Stream when creating a stage

What does this MR do?

This MR ensures that a GroupStage model is always associated with a ValueStream object. Since we have existing GroupStage records, we'll create a container ValueStream model (name: default).

The old URLs where the ValueStream parameter (value_stream_id) is not available is still active (and might be used until the FE part is ready). To support zero downtime deployment, we ensure that an associated ValueStream model is always present.

Post Migration:

1. Find all GroupStage records.
2. Associate it with a newly created ValueStream record (default).
3. Set NOT NULL constraint on group_value_stream_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
• [-] 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