Ensure EE files refer to Design Management models using top-level `::DesignManagement` namespace

What does this MR do?

This MR ensures that /ee classes refer to Design Management models using top-level ::DesignManagement:: namespace, rather than just DesignManagement::.

The models were moved from /ee to FOSS recently in !30015 (merged).

A test began failing in an MR I was preparing for moving the Design Management services !30961 (merged) due to:

NameError: uninitialized constant EE::DesignManagement::Design

This was because EE::Notable referred to DesignManagement::Design here:

https://gitlab.com/gitlab-org/gitlab/-/blob/e75b3b5aba5a36ac59f3b2b4891c6d1e13eb0b44/ee/app/models/concerns/ee/noteable.rb#L27

This changes all references to DesignManagement to use ::DesignManagement from classes within /ee.

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