feat(storage): remove deprecated legacy checksum options completely

[v-pratap]

This PR removes the deprecated DisableMD5Hash and DisableCrc32cChecksum completely from the codebase as requested for the next major release. This includes:

• Removing deprecated options from public headers (hashing_options.h).
• Removing legacy fallback parsing logic.
• Migrating all internal uses (tests, streambuf, validators) to use UploadChecksumValidationOption and DownloadChecksumValidationOption.

This also includes the missing checksum options migrations from parallel_upload.h and the benchmarks.

[gemini-code-assist]

Code Review

This pull request removes the deprecated DisableMD5Hash and DisableCrc32cChecksum options, replacing them with UploadChecksumValidationOption and DownloadChecksumValidationOption across the codebase, including tests and benchmarks. The review feedback highlights several style guide violations, such as lines exceeding the 80-character limit, excessive blank lines, and a broken Doxygen comment block. Additionally, it is recommended to factor out a repeated nested ternary expression in throughput_experiment.cc into a helper function to reduce duplication and improve readability.

[codecov]

Codecov Report

❌ Patch coverage is 78.87324% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.30%. Comparing base (f8a0cac) to head (f12e48c).

Files with missing lines	Patch %	Lines
.../cloud/storage/benchmarks/throughput_experiment.cc	0.00%	12 Missing ⚠️
.../cloud/storage/internal/hash_function_impl_test.cc	91.66%	1 Missing ⚠️
...ogle/cloud/storage/internal/hash_validator_test.cc	83.33%	1 Missing ⚠️
google/cloud/storage/testing/upload_hash_cases.cc	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16279      +/-   ##
==========================================
- Coverage   92.30%   92.30%   -0.01%     
==========================================
  Files        2223     2223              
  Lines      208086   207899     -187     
==========================================
- Hits       192074   191893     -181     
+ Misses      16012    16006       -6     

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