Deprecate DataClient public member function

[dbolduc]

What?: We want to deprecate and schedule for deletion these member functions: https://github.com/googleapis/google-cloud-cpp/blob/8e9b821839901e34d9c030171afea28dd7ce96b4/google/cloud/bigtable/data_client.h#L63-L84

Why?: We are removing these because:

• Only the client library should need this level of control over the gRPC channels.
• We want to perform work using the Connection layer (which offers customers better mocks, unified authentication credentials support, etc.). We do not want to maintain the legacy code path (these functions), when there is a preferred alternative. See #5934.

How?: We still call some of these. We can create private implementations where necessary. Our code can call the private implementations directly. The public member functions would also call the private implementations.

[dbolduc]

The functions are deprecated. They can be deleted (and the class can be simplified) on 2023-05-01.

[coryan]

These are deprecated, scheduled for removal around 2023-07.