Deprecate handwritten Pub/Sub admin API

[alevenberg]

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

• Move and replace handwritten Pub/Sub samples with the generated API
  • Topic Admin
  • Subscription Admin
• Point our google3 docs at the new samples
  • Topic Admin
  • Subscription Admin
• Move and replace handwritten Pub/Sub samples with the generated API in samples.cc
  • Topic Admin
  • Subscription Admin
• Move and replace handwritten Pub/Sub api in the publisher/subscriber integration tests (not admin)
  • Topic Admin
  • Subscription Admin
• Add @deprecated to the doxygen documentation for the old APIs
  • Topic Admin
  • Subscription Admin
• Add [[deprecated]] attributes to the old APIs.
  • Topic Admin
  • Subscription Admin
• Manually move the google3 dependents
  • TopicAdminClient - 3 references
  • SubscriptionAdminClient - 4 references - Blocked on Privacy Sandbox b/324627365
    • Update google cloud C++ library to version 2.22+ privacysandbox/data-plane-shared-libraries#4
• [optional] Omit the files we don't want people to use in the copybara import (cl/613286361)
  • Topic Admin
  • Subscription Admin
• Delete old code (hooray!) in Q4 2024
  • Remove all the admin files
  • Omit the admin rpcs from the generated data stub