Migrate to gRPC GA APIs

[tswast]

The exception types thrown are changing in the gRPC GA APIs compared to the beta APIs currently used.
grpc/grpc#7002 (comment)

To minimize the amount of code users of the Cloud Bigtable APIs will need to rewrite, we should migrate to the GA APIs ASAP.

[tseaver]

@tswast I'm fine with using the GA version of the API. It would be good the the gRPC team updated their examples to show us how.

[tseaver]

Via #2149.