cleanup(pubsub): add tags to topic admin samples

[alevenberg]

Part of #12987

This change is 

[snippet-bot]

Here is the summary of changes.

You are about to add 9 region tags.

• google/cloud/pubsub/samples/topic_admin_samples.cc:87, tag pubsub_quickstart_create_topic
• google/cloud/pubsub/samples/topic_admin_samples.cc:88, tag pubsub_create_topic
• google/cloud/pubsub/samples/topic_admin_samples.cc:112, tag pubsub_create_topic_with_schema
• google/cloud/pubsub/samples/topic_admin_samples.cc:143, tag pubsub_create_topic_with_schema_revisions
• google/cloud/pubsub/samples/topic_admin_samples.cc:214, tag pubsub_update_topic_schema
• google/cloud/pubsub/samples/topic_admin_samples.cc:245, tag pubsub_list_topics
• google/cloud/pubsub/samples/topic_admin_samples.cc:265, tag pubsub_list_topic_subscriptions
• google/cloud/pubsub/samples/topic_admin_samples.cc:299, tag pubsub_detach_subscription
• google/cloud/pubsub/samples/topic_admin_samples.cc:319, tag pubsub_delete_topic

You are about to delete 4 region tags.

• google/cloud/pubsub/samples/topic_admin_samples.cc:110, tag pubsub_create_topic_with_schema
• google/cloud/pubsub/samples/topic_admin_samples.cc:141, tag pubsub_create_topic_with_schema_revisions
• google/cloud/pubsub/samples/topic_admin_samples.cc:218, tag pubsub_update_topic_schema
• google/cloud/pubsub/samples/topic_admin_samples.cc:306, tag pubsub_detach_subscription

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d9843c5) 93.29% compared to head (267ce22) 93.28%.
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13590      +/-   ##
==========================================
- Coverage   93.29%   93.28%   -0.01%     
==========================================
  Files        2197     2197              
  Lines      192012   192009       -3     
==========================================
- Hits       179128   179124       -4     
- Misses      12884    12885       +1     

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