Cherrypick #8702 to v1.77.x

[easwars]

#8548 changed the grpc status code returned on missing grpc-status header to Internal. This PR changes it back to Unknown as that is the expected behavior.

See grpc/grpc@master/doc/statuscodes.md for more details. Here Unknown is defined as follows:

Unknown error. For example, this error may be returned when a Status value received 
from another address space belongs to an error space that is not known in this 
address space. Also errors raised by APIs that do not return enough error information
may be converted to this error.

RELEASE NOTES:

• transport/client : Return status code Unknown on missing grpc-status.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.21%. Comparing base (cadae08) to head (b3e21d8).
⚠️ Report is 1 commits behind head on v1.77.x.

Additional details and impacted files

@@             Coverage Diff             @@
##           v1.77.x    #8709      +/-   ##
===========================================
- Coverage    83.28%   83.21%   -0.07%     
===========================================
  Files          418      418              
  Lines        32347    32347              
===========================================
- Hits         26941    26919      -22     
- Misses        4029     4040      +11     
- Partials      1377     1388      +11     

Files with missing lines	Coverage Δ
internal/transport/http2_client.go	92.41% <100.00%> (+0.48%)	⬆️

... and 30 files with indirect coverage changes

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