cleanup: heed abseil-string-find-startswith clang-tidy

[devbww]

Some downstream users enable this clang-tidy, so we might as well take
its advice.

https://clang.llvm.org/extra/clang-tidy/checks/abseil/string-find-startswith.html

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This change is 

[google-cloud-cpp-bot]

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For commit: 77ddb1ba960c0e2b61c1168d99121733f47d4cbe

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[codecov]

Codecov Report

Merging #9416 (77ddb1b) into main (c23bcf2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #9416   +/-   ##
=======================================
  Coverage   94.61%   94.61%           
=======================================
  Files        1484     1484           
  Lines      136115   136116    +1     
=======================================
+ Hits       128779   128780    +1     
  Misses       7336     7336           

Impacted Files	Coverage Δ
...gle/cloud/pubsub/internal/exactly_once_policies.cc	100.00% <100.00%> (ø)
.../cloud/pubsub/internal/extend_leases_with_retry.cc	95.00% <100.00%> (ø)
google/cloud/pubsub/subscriber_connection_test.cc	97.24% <0.00%> (-0.69%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.00% <0.00%> (ø)
google/cloud/pubsub/samples/samples.cc	92.18% <0.00%> (+0.07%)	⬆️

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