impl(v3): remove backward compatibility cmake proto libs

[scotthart]

The iam protos were not included in this PR. They will be handled separately.

And fix quickstart.sh script.

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This change is 

[gemini-code-assist]

Summary of Changes

Hello @scotthart, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request focuses on refining the project's CMake build system by removing outdated backward-compatible proto library aliases, which simplifies dependencies and reduces technical debt. It also includes a minor but important fix to the quickstart.sh script for more efficient error handling. To ensure a smooth transition for users, the official v3 migration guide has been updated to clearly document these changes and provide necessary guidance.

Highlights

• CMake Backward Compatibility Removal: Deprecated and removed CMake logic for backward-compatible proto interface libraries, streamlining the build system. This change specifically excludes IAM protos, which will be addressed in a separate effort.
• Quickstart Script Fix: The quickstart.sh script's cleanup function was updated to conditionally search for core dump files only when the script exits with a non-zero status, preventing unnecessary searches on successful runs.
• Migration Guide Update: The v3 migration guide has been updated with a new section detailing the removed backward-compatible proto interface libraries and providing guidance on how to update CMakeLists.txt files to use the preferred proto library names.

Changelog

• ci/cloudbuild/builds/lib/quickstart.sh
  • Modified the cleanup function to only execute the find . -name '*core*' command if the script's exit status is not zero, improving efficiency.
• ci/verify_current_targets/CMakeLists.txt
  • Reduced the list of libraries checked for backward compatibility, now focusing solely on IAM-related protos as other backward-compatible proto libraries have been removed.
  • Updated the backwards_compat_proto_libraries list to reflect the removal of several deprecated proto library entries, retaining only IAM-specific ones.
• cmake/GoogleCloudCppLibrary.cmake
  • Removed the foreach loop and associated logic responsible for creating and managing backward-compatible proto interface libraries, aligning with the deprecation strategy.
• doc/v3-migration-guide.md
  • Added a new collapsible section under 'CMake' to document the removal of backward-compatible proto interface libraries.
  • Included a table mapping decommissioned proto libraries to their preferred modern equivalents, assisting users in updating their CMakeLists.txt files.

Activity

• The author, scotthart, initiated this pull request to address the removal of backward compatibility for CMake proto libraries and fix a script issue.
• The PR description clarifies that IAM protos are being handled separately and notes the inclusion of a fix for quickstart.sh.

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[gemini-code-assist]

Code Review

This pull request removes backward compatibility for several CMake proto libraries. The changes to the tests and the quickstart.sh script are good. However, there is a critical issue in cmake/GoogleCloudCppLibrary.cmake: the removal of the backward-compatibility logic will break the iam library, which is meant to be handled separately. I've recommended reverting that part of the change. Additionally, the migration guide in doc/v3-migration-guide.md is missing some of the decommissioned libraries in its summary table, which I've pointed out for correction.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.80%. Comparing base (e1d9216) to head (6eec408).
⚠️ Report is 2 commits behind head on prepare-for-v3.0.0.

Additional details and impacted files

@@                  Coverage Diff                   @@
##           prepare-for-v3.0.0   #15931      +/-   ##
======================================================
+ Coverage               92.79%   92.80%   +0.01%     
======================================================
  Files                    2332     2332              
  Lines                  214386   214386              
======================================================
+ Hits                   198946   198968      +22     
+ Misses                  15440    15418      -22     

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