[Key Vault] Re-generate libraries and release first beta for service version 7.6-preview.2

[gracewilcox]

Use the service specification added in this commit. Changes should only include two new KV Administration LRO endpoints: /prebackup and /prerestore, much akin to /backup and /restore.

[chlowell]

The TSP commit is Azure/azure-rest-api-specs@b8d26b0. New features vs. 7.5 are

• admin
  • prebackup
  • prerestore
• certs
  • preserveCertOrder option for create and import operations, field on CertificateOperation and CertificateBundle models
• keys
  • new values for signature and encryption algorithm enums
  • getKeyAttestation