Deprecate handwritten Pub/Sub admin API

From [go/cloud-cxx:pub-sub-admin-apis](http://goto.google.com/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

Start by tracing the four remaining SubscriptionAdminClient references, noting that this work is blocked on the Privacy Sandbox issue linked in the checklist. Then inspect the remaining admin files and generated data stub; done means moving the dependents, removing the old admin code, and omitting the admin RPCs.

Written by the indexing model from the issue text.