impl(generator): backwards compat using declarations are deprecated

[dbolduc]

Part of the work for #10170

In response to: #10463 (comment)

---

This change is 

[google-cloud-cpp-bot]

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For commit: 18b87a54a51607e6337f6faa48bb44cb6365ce6b

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[codecov]

Codecov Report

Base: 93.87% // Head: 93.88% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (18b87a5) compared to base (b191e83).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10466   +/-   ##
=======================================
  Coverage   93.87%   93.88%           
=======================================
  Files        1624     1624           
  Lines      147462   147462           
=======================================
+ Hits       138436   138449   +13     
+ Misses       9026     9013   -13     

Impacted Files	Coverage Δ
generator/internal/forwarding_client_generator.cc	0.00% <ø> (ø)
...erator/internal/forwarding_connection_generator.cc	0.00% <ø> (ø)
...nternal/forwarding_idempotency_policy_generator.cc	0.00% <ø> (ø)
...r/internal/forwarding_mock_connection_generator.cc	0.00% <ø> (ø)
generator/internal/forwarding_options_generator.cc	0.00% <ø> (ø)
...sub/internal/batching_publisher_connection_test.cc	99.62% <0.00%> (+0.18%)	⬆️
google/cloud/storage/parallel_upload.cc	98.62% <0.00%> (+0.34%)	⬆️
...ud/spanner/integration_tests/client_stress_test.cc	84.86% <0.00%> (+0.65%)	⬆️
...cloud/pubsub/internal/subscription_session_test.cc	98.66% <0.00%> (+0.66%)	⬆️
google/cloud/bigtable/async_read_stream_test.cc	97.99% <0.00%> (+0.66%)	⬆️
... and 2 more

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