Automatically set position for VSA stages

What does this MR do?

This MR updates the value stream services to automatically set relative_position for the nested stages (accepts_nested_attributes) based on the order we receive them from the FE.

Additionally I remove the extra sorting on FE side. It's not necessary to sort on the FE because the stages are already sorted on the BE (API).

Note The FE changes that are using the BE functionality is behind FF: value_stream_analytics_extended_form

How to test it:

Make sure you're on ultimate. Enable the value_stream_analytics_extended_form feature flag.

1. Create a new group
2. Navigate to Analytics / Value Stream
3. In the drop down add a new value stream, add two stages and save.
4. Edit the value stream and move the stages and save
5. The page should show the stages in the correct order.

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