cleanup(pubsub): switch over to new Make*Connection functions in tests, benchmarks, examples

[dbolduc]

Part of the work for #6306

The only remaining calls to the old functions are captured in: #7347 and #7344

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This change is 

[google-cloud-cpp-bot]

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For commit: a146309adca9e90ac0a7c2183c190f18068488e5

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[codecov]

Codecov Report

Merging #7349 (a146309) into main (7414d8a) will decrease coverage by 0.01%.
The diff coverage is 96.80%.

@@            Coverage Diff             @@
##             main    #7349      +/-   ##
==========================================
- Coverage   93.65%   93.63%   -0.02%     
==========================================
  Files        1352     1352              
  Lines      117092   117092              
==========================================
- Hits       109657   109641      -16     
- Misses       7435     7451      +16     

Impacted Files	Coverage Δ
google/cloud/pubsub/benchmarks/throughput.cc	93.38% <91.30%> (+0.06%)	⬆️
google/cloud/pubsub/samples/samples.cc	92.03% <96.55%> (+0.06%)	⬆️
google/cloud/pubsub/benchmarks/endurance.cc	88.85% <100.00%> (ø)
...integration_tests/schema_admin_integration_test.cc	100.00% <100.00%> (ø)
...b/integration_tests/subscriber_integration_test.cc	98.20% <100.00%> (-0.01%)	⬇️
...ation_tests/subscription_admin_integration_test.cc	95.26% <100.00%> (-0.26%)	⬇️
.../integration_tests/topic_admin_integration_test.cc	97.77% <100.00%> (+0.05%)	⬆️
...ogle/cloud/pubsub/samples/pubsub_samples_common.cc	100.00% <100.00%> (ø)
google/cloud/pubsub/schema_admin_connection.cc	90.12% <0.00%> (-7.41%)	⬇️
google/cloud/pubsub/topic_admin_connection.cc	84.51% <0.00%> (-3.88%)	⬇️
... and 7 more

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