Fixes for design reordering

What does this MR do?

Some fixes for #34382 (closed):

• Insert new designs at the end.
  • This was already the case in the backend, but not on the frontend.
  • We also don't need to initialize a position for new designs, as they're already sorted last.
• Remove background migration.
  • This is not needed anymore, since we're now backfilling the design positions immediately before moving a design.
• Enable the feature flag by default.

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

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• 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