Update DevCenter to use CustomerManagedKeyEncryptionV4 from Foundations

Replace direct usage of internal CommonTypes.CustomerManagedKeyEncryption with
Azure.ResourceManager.Foundations.CustomerManagedKeyEncryptionV4 in DevCenter spec.
Add clientName decorator in back-compatible.tsp to strip V4 suffix for OpenAPI
compatibility.

Depends on: Azure/typespec-azure#4627

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: markcowl