doc(spanner): convert Spanner samples to use the generated admin APIs

[devbww]

This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #7311 (bbab073) into main (480cd16) will decrease coverage by 0.03%.
The diff coverage is 40.46%.

@@            Coverage Diff             @@
##             main    #7311      +/-   ##
==========================================
- Coverage   93.69%   93.65%   -0.04%     
==========================================
  Files        1347     1349       +2     
  Lines      116534   116605      +71     
==========================================
+ Hits       109185   109207      +22     
- Misses       7349     7398      +49     

Impacted Files	Coverage Δ
google/cloud/spanner/samples/samples.cc	62.83% <40.46%> (-0.62%)	⬇️
google/cloud/spanner/instance_admin_connection.cc	91.87% <0.00%> (-4.38%)	⬇️
...bigtable/examples/bigtable_hello_instance_admin.cc	81.31% <0.00%> (-2.20%)	⬇️
...le/cloud/internal/default_completion_queue_impl.cc	97.00% <0.00%> (-0.60%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.78% <0.00%> (-0.25%)	⬇️
google/cloud/spanner/admin/database_admin_client.h	50.00% <0.00%> (ø)
google/cloud/spanner/admin/instance_admin_client.h	50.00% <0.00%> (ø)
google/cloud/pubsub/samples/samples.cc	92.04% <0.00%> (+0.07%)	⬆️
google/cloud/grpc_error_delegate.cc	100.00% <0.00%> (+4.16%)	⬆️

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