Deprecate handwritten Pub/Sub admin API

[alevenberg]

From go/cloud-cxx:pub-sub-admin-apis

• [x] Move and replace handwritten Pub/Sub samples with the generated API
  • [x] Topic Admin
  • [x] Subscription Admin
• [x] Point our google3 docs at the new samples
  • [x] Topic Admin
  • [x] Subscription Admin
• [x] Move and replace handwritten Pub/Sub samples with the generated API in samples.cc
  • [x] Topic Admin
  • [x] Subscription Admin
• [x] Move and replace handwritten Pub/Sub api in the publisher/subscriber integration tests (not admin)
  • [x] Topic Admin
  • [x] Subscription Admin
• [x] Add @deprecated to the doxygen documentation for the old APIs
  • [x] Topic Admin
  • [x] Subscription Admin
• [x] Add [[deprecated]] attributes to the old APIs.
  • [x] Topic Admin
  • [x] Subscription Admin
• [ ] Manually move the google3 dependents
  • [x] TopicAdminClient - 3 references
  • [ ] SubscriptionAdminClient - 4 references - Blocked on Privacy Sandbox b/324627365
    • https://github.com/privacysandbox/data-plane-shared-libraries/issues/4
• [x] [optional] Omit the files we don't want people to use in the copybara import (cl/613286361)
  • [x] Topic Admin
  • [x] Subscription Admin
• [ ] Delete old code (hooray!) in Q4 2024
  • [ ] Remove all the admin files
  • [ ] Omit the admin rpcs from the generated data stub