chore: release notes for v0.18.x

[devbww]

Also update the ABI dumps.

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This change is 

[coryan]

Reviewed 4 of 4 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @devjgm, @mr-salty, and @scotthart)

[codecov]

Codecov Report

Merging #3368 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3368      +/-   ##
==========================================
- Coverage   89.36%   89.35%   -0.02%     
==========================================
  Files         461      461              
  Lines       41947    41944       -3     
==========================================
- Hits        37488    37479       -9     
- Misses       4459     4465       +6

Impacted Files	Coverage Δ
...oogle/cloud/bigtable/internal/unary_client_utils.h	82.14% <0%> (-17.86%)	⬇️
google/cloud/storage/internal/hash_validator.h	70% <0%> (-10%)	⬇️
...le/cloud/bigtable/internal/async_retry_unary_rpc.h	81.48% <0%> (-3.71%)	⬇️
google/cloud/storage/well_known_headers.h	88.88% <0%> (-3.71%)	⬇️
google/cloud/storage/internal/generic_request.h	84.61% <0%> (-2.57%)	⬇️
google/cloud/storage/well_known_parameters.h	79.34% <0%> (-2.18%)	⬇️
...e/benchmarks/storage_parallel_uploads_benchmark.cc	70% <0%> (+0.5%)	⬆️
google/cloud/storage/parallel_upload.h	96.21% <0%> (+0.72%)	⬆️
google/cloud/storage/parallel_upload.cc	99% <0%> (+0.98%)	⬆️
...le/cloud/storage/internal/curl_download_request.cc	76.43% <0%> (+1.43%)	⬆️

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