Revert "chore(deps): update dependency com_github_grpc_grpc to v1.38.1"

[devjgm]

Reverts #6826

Rolling this back because grpc 1.38.1 breaks our builds that use pkg-config. See also #6840

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[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #6841 (34388e6) into main (c0be918) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #6841      +/-   ##
==========================================
- Coverage   95.04%   95.04%   -0.01%     
==========================================
  Files        1264     1264              
  Lines      113312   113312              
==========================================
- Hits       107694   107693       -1     
- Misses       5618     5619       +1     

Impacted Files	Coverage Δ
google/cloud/spanner/internal/spanner_stub.cc	76.41% <0.00%> (-0.95%)	⬇️
google/cloud/bigtable/async_read_stream_test.cc	97.32% <0.00%> (-0.67%)	⬇️
google/cloud/pubsub/samples/samples.cc	91.75% <0.00%> (+0.07%)	⬆️
...le/cloud/storage/internal/curl_download_request.cc	81.41% <0.00%> (+0.44%)	⬆️
google/cloud/internal/async_rpc_details.h	100.00% <0.00%> (+5.88%)	⬆️

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