chore(storage): Remove gRPC backwards compatibility shims

[Pulkit0110]

Since gRPC is now GA, removing the backwards compatibility shims for gRPC.

• storage_experimental namespace is still present in grpc_plugin.h as it has the members related to gRPC metrics like EnableGrpcMetricsOption and GrpcMetricsPeriodOption. I'm not sure whether this is completely implemented.

• Simplify MakeGrpcClient(): drop the UseRest() branch and ENV-var fallback so it unconditionally returns a gRPC-backed Client. Previously MakeGrpcClient() could return JSON client by comparing the ENV variable. Now it is removed so that we've a clear separation of gRPC and JSON clients.

• Refactored unit tests as per the new behavior.

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This change is 

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 92.93%. Comparing base (67bbff6) to head (102cd0c). :warning: Report is 75 commits behind head on prepare-for-v3.0.0.

Additional details and impacted files

@@                  Coverage Diff                   @@
##           prepare-for-v3.0.0   #15146      +/-   ##
======================================================
- Coverage               92.93%   92.93%   -0.01%     
======================================================
  Files                    2392     2392              
  Lines                  215083   215022      -61     
======================================================
- Hits                   199887   199822      -65     
- Misses                  15196    15200       +4     

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[ddelgrosso1]

@Pulkit0110 can we fix up the errors and take this out of draft if it is ready to review?