fix(otel): restore compatibility with otel 1.9.1

[dbolduc]

Part of the work for #12991

Instead of changing the values of these semantic convention variables, OTel deprecates them, and tells you to use the new name. That is weird, but whatever.

Let's not mess with any #if OPENTELEMETRY_CPP_VERSIONs. Let's just use the values du jour directly.

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This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6a3b9ae) 93.60% compared to head (631c1d6) 93.59%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12992      +/-   ##
==========================================
- Coverage   93.60%   93.59%   -0.01%     
==========================================
  Files        2069     2069              
  Lines      180941   180945       +4     
==========================================
- Hits       169361   169359       -2     
- Misses      11580    11586       +6     

Files	Coverage Δ
google/cloud/internal/grpc_opentelemetry.cc	88.00% <ø> (ø)
google/cloud/internal/grpc_opentelemetry_test.cc	100.00% <ø> (ø)
google/cloud/internal/rest_opentelemetry.cc	93.10% <100.00%> (ø)
google/cloud/internal/rest_opentelemetry_test.cc	100.00% <ø> (ø)
google/cloud/internal/tracing_http_payload_test.cc	90.19% <100.00%> (ø)
google/cloud/internal/tracing_rest_client.cc	89.47% <100.00%> (+0.46%)	⬆️
google/cloud/internal/tracing_rest_client_test.cc	100.00% <ø> (ø)
...oogle/cloud/internal/tracing_rest_response_test.cc	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes

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