Make DesignManagement prefixes consistent

What does this MR do and why?

Make DesignManagement prefixes consistent

• A DesignManagement class in FOSS (at time of writing, all of them) referenced from EE should use ::DesignManagement.
• A DesignManagement class in FOSS referenced from FOSS should not be prefixed with :: and instead use just DesignManagement, where possible (it may be necessary to always prefix in GraphQL files !30961 (comment 340777574) and inside the API::Entities namespace to avoid naming collission with API::Entities::DesignManagement::Design).

Issue: #217420 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #217420 (closed)