[INTERNAL CHANGE]: Update to grpc-swift 1.7.1

[bauer-andreas]

Problem

The GRPCMigrator currently relies and generates code for grpc-swift version 1.6.0-async-await.1.
grpc-swift 1.7.1-async-await.2 was recently updated and includes several breaking changes.
We are affected by the following change:

• Use requests/requestStream and responses/responseStream consistently. (Use {request,response}{s,stream} consistently grpc/grpc-swift#1320)

The other breaking change doesn't affect us.

Solution

Update the GRPCCall generation to use the updated external parameters.

Additional context

No response

Code of Conduct

• I agree to follow this project's Code of Conduct and Contributing Guidelines